Kernel/LibC: Add initial pseudo terminal support

This patch implements posix_openpt() and ptsname()

grantpt() and unlockpt() are left in LibC as stubs, as posix_openpt
currently does all of the needed work.

kernel/include/kernel/Device/DeviceNumbers.h

@@ -9,7 +9,7 @@ namespace Kernel
 	{
 		Framebuffer = 1,
 		TTY,
-		Serial,
+		PTSMaster,
 		Null,
 		Zero,
 		Debug,