Fix 'starttls.el' on native Windows Emacs with gnutls-cli from Cygwin.
'gnutls-cli' run with '-s' opt and process wait for SIGALRM.
But build-in native Emacs 'kill' command can not send such Cygwin
specific sygnal. So 'starttls-negotiate-gnutls' function infinitely
wait for 'gnutls-cli' output.