relibc: implement cfgetispeed/cfsetispeed on Redox

Replaces the four hard stubs (cfgetispeed, cfgetospeed,
cfsetispeed, cfsetospeed) that returned either 0 or EINVAL
unconditionally on Redox. The functions were tracked in the
Round 7 audit as a 'medium' severity gap (serial tools like
minicom, screen, cu that query baud rate would always see 0).

The fix:
* Adds two non-POSIX extension fields to the Redox termios
  struct: __c_ispeed and __c_ospeed (both speed_t, default 0).
  Documented as 'non-POSIX; B0..=B4000000' per BSD conventions.
* cfgetispeed now reads termios.__c_ispeed (was 0)
* cfgetospeed now reads termios.__c_ospeed (was 0)
* cfsetispeed writes termios.__c_ispeed for valid speeds
  (B0..=B38400, B57600..=B4000000); EINVAL otherwise
  (was EINVAL for all speeds on Redox)
* cfsetospeed writes termios.__c_ospeed for valid speeds
  (was EINVAL for all speeds on Redox)

The added fields are placed after c_cc to match BSD/glibc
struct termios layout ordering. Redox has no established
userspace ABI for termios yet, so the layout extension is
safe. Applications that only call cfget*/cfset*baud
functions (and not direct field access) will now get real
baud-rate roundtripping.

POSIX compliance: the functions now correctly return the
stored speed on get, store the requested speed on set,
and return -1 with errno=EINVAL for out-of-range speeds.

src/header/termios/mod.rs

@@ -91,6 +91,10 @@ pub struct termios {
     pub c_lflag: tcflag_t,
     /// Control characters.
     pub c_cc: [cc_t; NCCS],
+    /// Input baud rate (non-POSIX; B0..=B4000000).
+    pub __c_ispeed: speed_t,
+    /// Output baud rate (non-POSIX; B0..=B4000000).
+    pub __c_ospeed: speed_t,
 }
 
 /// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/tcgetattr.html>.
@@ -171,8 +175,7 @@ pub unsafe extern "C" fn cfgetispeed(termios_p: *const termios) -> speed_t {
     }
     #[cfg(target_os = "redox")]
     {
-        //TODO
+        unsafe { (*termios_p).__c_ispeed }
-        0
     }
 }
 
@@ -193,8 +196,7 @@ pub unsafe extern "C" fn cfgetospeed(termios_p: *const termios) -> speed_t {
     }
     #[cfg(target_os = "redox")]
     {
-        //TODO
+        unsafe { (*termios_p).__c_ospeed }
-        0
     }
 }
 
@@ -208,8 +210,6 @@ pub unsafe extern "C" fn cfgetospeed(termios_p: *const termios) -> speed_t {
 #[unsafe(no_mangle)]
 pub unsafe extern "C" fn cfsetispeed(termios_p: *mut termios, speed: speed_t) -> c_int {
     match speed as usize {
-        // TODO redox impl
-        #[cfg(target_os = "linux")]
         B0..=B38400 | B57600..=B4000000 => {
             unsafe { (*termios_p).__c_ispeed = speed };
             0
@@ -231,8 +231,6 @@ pub unsafe extern "C" fn cfsetispeed(termios_p: *mut termios, speed: speed_t) ->
 #[unsafe(no_mangle)]
 pub unsafe extern "C" fn cfsetospeed(termios_p: *mut termios, speed: speed_t) -> c_int {
     match speed as usize {
-        // TODO redox impl
-        #[cfg(target_os = "linux")]
         B0..=B38400 | B57600..=B4000000 => {
             unsafe { (*termios_p).__c_ospeed = speed };
             0