DAOS-14826 client: reserve low fds (#13532)

Reserve low fd to avoid daos network contexts use them.
Fix a corner case in fstatat() to mimic libc and add unit test.
Rely on libc dup2() to get desired fd to avoid error "failed to get the desired fd in dup2()".
Use atomic APIs to access daos_inited.
Move pthread_atfork() inside locked region to avoid multiple execution.
Call glibc chdir() to update process's cwd in kernel state.
Introduce fd duplication reference count to handle complex situation

Add a unit test to make sure daos does not use low fd.

Signed-off-by: Lei Huang <[email protected]>