Consider replacing the implementation with just the ctermid syscall

[AlexTMjugador]

The POSIX standard defines the ctermid C function to get the file path of the process controlling terminal. All Unix-like OSes are mostly POSIX compliant, so this function can be assumed to be available on every Unix-like target.

Therefore, this crate could be refactored to just be a nice, safe facade to that C function, to save quite a bit of complexity with build scripts and be even more cross-platform than it currently is.

For a project of mine, I've came up with the following function that you can use as you like:

/// Returns a file path to the controlling terminal of this process. If this
/// process has no controlling terminal or an error happens, `None` will
/// be returned.
fn ctermid() -> Option<String> {
	use std::ffi::CString;
	use std::os::raw::{c_char, c_uchar};

	extern "C" {
		/// `char* ctermid(char* s)`, from `#include <stdio.h>`.
		///
		/// Documentation: <https://pubs.opengroup.org/onlinepubs/9699919799/functions/ctermid.html>
		fn ctermid(s: *mut c_char) -> *mut c_char;
	}

	// SAFETY: system calls are unsafe. ctermid is required by the standard to populate the passed
	// pointer with a valid string, always. If some error happens, then the string is empty, but the
	// pointer is valid. Because we bring our own buffer, safe Rust is in full control of the data
	// lifetime. L_ctermid is 9 on Linux, but allocating space for 256 characters should guarantee
	// that the buffer size is always greater than L_ctermid. c_uchar has the same memory layout
	// than c_char and are interchangeable
	#[allow(unsafe_code)]
	let path = unsafe {
		let mut path_buf = vec![0 as c_uchar; 256];

		ctermid(path_buf.as_mut_ptr() as *mut c_char);

		// CString requires that the Vec exactly contains a C string, with no NUL bytes
		let mut found_nul = false;
		path_buf.retain(|byte| {
			let is_not_first_nul = *byte != 0 && !found_nul;
			found_nul = found_nul || *byte == 0;
			is_not_first_nul
		});

		CString::new(path_buf)
	};

	path.ok()
		.and_then(|path_cstring| path_cstring.into_string().ok())
		.filter(|path| !path.is_empty())
}

Thank you for making this useful library, by the way!