bug#49524: 28.0.50; make-serial-process is not portable

[Eli Zaretskii <eliz@gnu.org>]

> From: Ken Brown <kbrown@cornell.edu>
> Date: Sun, 11 Jul 2021 11:24:58 -0400
> 
> Fmake_serial_process calls Fserial_process_configure, which calls 
> serial_configure, which calls cfsetspeed with the speed argument equal to the 
> numerical baud rate (e.g., 9600).  But the documentation of cfsetspeed says that 
> the speed argument must be one of the Bnnn constants defined in termios.h (e.g., 
> B9600).  See, for example,
> 
>    https://man7.org/linux/man-pages/man3/termios.3.html
> 
> This incorrect call of cfsetspeed happens to succeed on GNU/Linux because 
> glibc's cfsetspeed allows the argument to be the numerical baud rate, which it 
> converts to the appropriate Bnnn constant.  But I don't think emacs should be 
> relying on this undocumented behavior.  In particular, this doesn't work on 
> Cygwin.  And it wouldn't even work on GNU/Linux if emacs used the cfsetspeed 
> replacement defined in sysdep.c instead of glibc's cfsetspeed.
> 
> I think the way to fix this is to imitate the glibc code that converts the baud 
> rate to a Bnnn constant, but maybe someone has a better idea.

Converting in sysdep.c:serial_configure sounds TRT to me.