[libc] Implement fchownat in unistd (#227974)

Implement the standard POSIX.1-2008 / POSIX.1-2024 function `fchownat`
in `<unistd.h>`, bringing the POSIX `*at` family of functions in
`<unistd.h>` to 100% completion.

Fixes #227848

Assisted by Antigravity and reviewed by Aman Maurya

GitOrigin-RevId: 4428e953e2a5da4f870d819a30de3aaba08cf0af