Cygwin: cfsetspeed: allow speed to be a numerical baud rate

Cygwin: cfsetspeed: allow speed to be a numerical baud rate

The Linux man page for cfsetspeed(3) specifies that the speed argument
must be one of the constants Bnnn (e.g., B9600) defined in termios.h.
But Linux in fact allows the speed to be the numerical baud rate
(e.g., 9600).  For consistency with Linux, we now do the same.

Addresses: https://cygwin.com/pipermail/cygwin/2021-July/248887.html