termios: Added cfsetspeed function.

BSD extension to set both the input and output speed of a termios
structure.

Fixes #18220.

Change-Id: I8c4a06b4be4aa55b8ce35cb7f62552fc47a8e8d0
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6049
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>