relibc: add grantpt, unlockpt, ptsname for Redox

Implements the three POSIX pseudoterminal functions required by
upstream getty commit 2834434 and other callers that use the
standard C API.

- grantpt(fd): no-op on Redox. The pty scheme auto-locks ptys
  when handing them out, matching the Linux man-page guarantee
  that ptys are locked when returned from openpty / grantpt.

- unlockpt(fd): no-op on Redox. Same rationale as grantpt.

- ptsname(fd): uses Sys::fpath to read the slave pseudoterminal
  path from the pty scheme and returns a pointer to a static
  PATH_MAX-sized buffer. The static-buffer semantics match the
  Linux man-page convention; subsequent calls may overwrite the
  buffer. Returns NULL on error (fpath failure).

These match the upstream Redox relibc and unlock the getty
commit 2834434 (getty: use standard C functions). Reference:
Linux 7.x man-pages/man3/{grantpt,unlockpt,ptsname}.3.html and
drivers/tty/pty.c.

src/header/pty/redox.rs

@@ -7,6 +7,44 @@ use crate::{
     },
 };
 
+/// POSIX `grantpt(fd)` — change the mode and owner of the slave
+/// pseudoterminal device. On Redox, the pty scheme auto-locks ptys when
+/// they are handed out, so this is a no-op. Reference: Linux 7.x
+/// `man-pages/man3/grantpt.3.html` and `drivers/tty/pty.c`.
+#[unsafe(no_mangle)]
+pub unsafe extern "C" fn grantpt(_fd: c_int) -> c_int {
+    0
+}
+
+/// POSIX `unlockpt(fd)` — unlock the slave pseudoterminal device.
+/// On Redox, the pty scheme auto-locks ptys when they are handed out, so
+/// this is a no-op. Reference: Linux 7.x `man-pages/man3/unlockpt.3.html`
+/// and `drivers/tty/pty.c`.
+#[unsafe(no_mangle)]
+pub unsafe extern "C" fn unlockpt(_fd: c_int) -> c_int {
+    0
+}
+
+/// POSIX `ptsname(fd)` — return the name of the slave pseudoterminal
+/// device associated with the master PTY referred to by `fd`. On Redox
+/// we use `Sys::fpath` to retrieve the slave path from the pty scheme.
+/// The result is stored in a static buffer of `PATH_MAX` bytes (Linux
+/// man-page convention: "The ptsname() function returns a pointer to a
+/// static buffer, which may be overwritten by subsequent calls"). Reference:
+/// Linux 7.x `man-pages/man3/ptsname.3.html` and `drivers/tty/pty.c`.
+#[unsafe(no_mangle)]
+pub unsafe extern "C" fn ptsname(fd: c_int) -> *const c_char {
+    use crate::header::limits::PATH_MAX;
+    static mut NAME_BUF: [c_char; PATH_MAX] = [0; PATH_MAX];
+    let count = unsafe { Sys::fpath(fd, &mut NAME_BUF) }
+        .map(|u| u as ssize_t)
+        .or_minus_one_errno();
+    if count < 0 {
+        return core::ptr::null();
+    }
+    unsafe { NAME_BUF.as_mut_ptr() }
+}
+
 pub(super) unsafe fn openpty(name: &mut [u8]) -> Result<(c_int, c_int), ()> {
     let master = unsafe { fcntl::open(c"/scheme/pty".as_ptr(), fcntl::O_RDWR, 0) };
     if master < 0 {