cfsetspeed is not consistent with Linux

While investigating an emacs bug (https://debbugs.gnu.org/cgi/bugreport.cgi?bug=49524), I noticed a difference in the behavior of cfsetspeed(3) on Cygwin and Linux. I'm not sure we should "fix" this, because Cygwin's behavior is consistent with the Linux man page, and Linux's behavior is not.

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