Add termios constant for VDISABLE

_POSIX_VDISABLE is an extension that disables terminal special
characters.

See:
* redox-os/termios!3
* redox-os/base!27

src/header/termios/linux.rs

@@ -111,3 +111,7 @@ pub const NOFLSH: usize = 0o000_200;
 pub const TOSTOP: usize = 0o000_400;
 pub const IEXTEN: usize = 0o100_000;
 /* } c_lflag */
+
+// POSIX extensions
+/// Sentinel value to disable a control char.
+pub const _POSIX_VDISABLE: u8 = 0;