feat: detect nmap for default scripts

[PsypherPunk]

If using a the equivalent of --scripts default, we're currently defaulting to nmap. However, as per #691, if it's not there, RustScan errors after completing the port-scan.

Making use of which to verify that an nmap binary can be found; this looks like:

❯ ./target/debug/rustscan --addresses 127.0.0.1 --scripts default
[!] Initiating scripts failed!
nmap: command not found. See <https://nmap.org/download>

1. is this something we actually want?
   • alternatively, we could log and disable the scripting engine.
2. if so, is the above adequate messaging?
3. although I can verify the behaviour, how can we wrap this in a unit test?

closes #691

[bee-san]

Becuase the script fails, I would maybe not use which and instead handle it in the code where it fails. So if it tries to run nmap, and nmap returns with status code > 0, we can write "Rustscan currently uses nmap...." 🤔

is this something we actually want?

Yes, my plan is to get rid of nmap in the future though :)

I am actually thinking of rewriting the script engine to also work with Rust files, and to turn some nmap scripts (the open source ones) into straight Rust code which should be fun.

if so, is the above adequate messaging?

"RustScan currently uses nmap for scripts, and nmap is not found. You can download it here {URL}"

although I can verify the behaviour, how can we wrap this in a unit test?

Not a unit test, but put it here:
https://github.com/RustScan/RustScan/blob/master/tests/

See timelimit.rs in that folder. It runs rustscan as a process just to see what the output is.

1. run as process
2. see output
3. check to see if output is fail state or not (os.status_code or something)

Because CI runs on 3 OS's, this behaviour and use of which will run on all 3.

[PsypherPunk]

Hmmm…I could reasonably do this by just checking for the right error code:

let es = match status.code() {
    Some(127) => return Err(anyhow!("RustScan currently uses nmap for scripts, and nmap is not found. You can download it here: https://nmap.org/download")),
    Some(code) => code,
    …

…but by that point we don't know that we're specifically running nmap, just that the executable in question couldn't be found.

Unless I'm missing something, this might be worth closing if there's a planned rewrite in the future.