implemented I/O safety traits and bump nix to 0.29

[solomoncyj]

Is your feature request related to a problem? Please describe.
in the nix 0.29 crates, the project cannot be built due the following errors, due to the enforcements of /O safety traits
https://download.copr.fedorainfracloud.org/results/solomoncyj/rust/fedora-rawhide-aarch64/08159687-rust-portable-pty/builder-live.log.gz

Compiling filedescriptor v0.8.2
     Running `/usr/bin/rustc --crate-name filedescriptor --edition=2018 /usr/share/cargo/registry/filedescriptor-0.8.2/src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --crate-type lib --emit=dep-info,metadata,link -C opt-level=3 -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2 --check-cfg 'cfg(docsrs)' --check-cfg 'cfg(feature, values())' -C metadata=cf2838f63fefe3c3 -C extra-filename=-cf2838f63fefe3c3 --out-dir /builddir/build/BUILD/rust-portable-pty-0.8.1-build/portable-pty-0.8.1/target/rpm/deps -L dependency=/builddir/build/BUILD/rust-portable-pty-0.8.1-build/portable-pty-0.8.1/target/rpm/deps --extern libc=/builddir/build/BUILD/rust-portable-pty-0.8.1-build/portable-pty-0.8.1/target/rpm/deps/liblibc-47e6d11bfd847524.rmeta --extern thiserror=/builddir/build/BUILD/rust-portable-pty-0.8.1-build/portable-pty-0.8.1/target/rpm/deps/libthiserror-9d9875ce5cfdfb82.rmeta --cap-lints allow -Copt-level=3 -Cdebuginfo=2 -Ccodegen-units=1 -Cstrip=none -Cforce-frame-pointers=yes -Clink-arg=-specs=/usr/lib/rpm/redhat/redhat-package-notes --cap-lints=warn`
   Compiling portable-pty v0.8.1 (/builddir/build/BUILD/rust-portable-pty-0.8.1-build/portable-pty-0.8.1)
     Running `/usr/bin/rustc --crate-name portable_pty --edition=2018 src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --crate-type lib --emit=dep-info,metadata,link -C opt-level=3 -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2 --cfg 'feature="default"' --check-cfg 'cfg(docsrs)' --check-cfg 'cfg(feature, values("default", "serde", "serde_derive", "serde_support"))' -C metadata=7af3effd4ac531b0 -C extra-filename=-7af3effd4ac531b0 --out-dir /builddir/build/BUILD/rust-portable-pty-0.8.1-build/portable-pty-0.8.1/target/rpm/deps -L dependency=/builddir/build/BUILD/rust-portable-pty-0.8.1-build/portable-pty-0.8.1/target/rpm/deps --extern anyhow=/builddir/build/BUILD/rust-portable-pty-0.8.1-build/portable-pty-0.8.1/target/rpm/deps/libanyhow-ba7a49b18d361cb0.rmeta --extern downcast_rs=/builddir/build/BUILD/rust-portable-pty-0.8.1-build/portable-pty-0.8.1/target/rpm/deps/libdowncast_rs-a83f85380c6eb557.rmeta --extern filedescriptor=/builddir/build/BUILD/rust-portable-pty-0.8.1-build/portable-pty-0.8.1/target/rpm/deps/libfiledescriptor-cf2838f63fefe3c3.rmeta --extern libc=/builddir/build/BUILD/rust-portable-pty-0.8.1-build/portable-pty-0.8.1/target/rpm/deps/liblibc-47e6d11bfd847524.rmeta --extern log=/builddir/build/BUILD/rust-portable-pty-0.8.1-build/portable-pty-0.8.1/target/rpm/deps/liblog-1927702d3a3787c8.rmeta --extern nix=/builddir/build/BUILD/rust-portable-pty-0.8.1-build/portable-pty-0.8.1/target/rpm/deps/libnix-8bf9f532a7732807.rmeta --extern serial=/builddir/build/BUILD/rust-portable-pty-0.8.1-build/portable-pty-0.8.1/target/rpm/deps/libserial-8c85a65000a910d1.rmeta --extern shell_words=/builddir/build/BUILD/rust-portable-pty-0.8.1-build/portable-pty-0.8.1/target/rpm/deps/libshell_words-6fe34738507834dd.rmeta -Copt-level=3 -Cdebuginfo=2 -Ccodegen-units=1 -Cstrip=none -Cforce-frame-pointers=yes -Clink-arg=-specs=/usr/lib/rpm/redhat/redhat-package-notes --cap-lints=warn`
warning: unexpected `cfg` condition value: `cargo-clippy`
  --> src/unix.rs:31:20
   |
31 |         #[cfg_attr(feature = "cargo-clippy", allow(clippy::unnecessary_mut_passed))]
   |                    ^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: expected values for `feature` are: `default`, `serde`, `serde_derive`, and `serde_support`
   = help: consider adding `cargo-clippy` as a feature in `Cargo.toml`
   = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html> for more information about checking conditional configuration
   = note: `#[warn(unexpected_cfgs)]` on by default

warning: unexpected `cfg` condition value: `cargo-clippy`
   --> src/unix.rs:228:32
    |
228 |                     #[cfg_attr(feature = "cargo-clippy", allow(clippy::cast_lossless))]
    |                                ^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: expected values for `feature` are: `default`, `serde`, `serde_derive`, and `serde_support`
    = help: consider adding `cargo-clippy` as a feature in `Cargo.toml`
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html> for more information about checking conditional configuration

error[E0277]: the trait bound `i32: AsFd` is not satisfied
   --> src/unix.rs:340:38
    |
340 |         nix::sys::termios::tcgetattr(self.fd.0.as_raw_fd()).ok()
    |         ---------------------------- ^^^^^^^^^^^^^^^^^^^^^ the trait `AsFd` is not implemented for `i32`
    |         |
    |         required by a bound introduced by this call
    |
    = help: the following other types implement trait `AsFd`:
              &T
              &mut T
              Arc<T>
              BorrowedFd<'_>
              Box<T>
              ChildStderr
              ChildStdin
              ChildStdout
            and 21 others
note: required by a bound in `nix::sys::termios::tcgetattr`
   --> /usr/share/cargo/registry/nix-0.29.0/src/sys/termios.rs:840:22
    |
840 | pub fn tcgetattr<Fd: AsFd>(fd: Fd) -> Result<Termios> {
    |                      ^^^^ required by this bound in `tcgetattr`

For more information about this error, try `rustc --explain E0277`.
warning: `portable-pty` (lib) generated 2 warnings
error: could not compile `portable-pty` (lib) due to 1 previous error; 2 warnings emitted

Describe the solution you'd like
please update the relevent code in portable-pty

Additional context
(sunfishcode/io-lifetimes#38 for more context around I/O safety)