Don't call com_setup() unless an alternate [COM:x] section is loaded

Apparently com_setup() (which just calls tcsetattr()) will cause an attached modem (with a caller) to disconnect the caller. So you can't do this with a live modem. Leave the hack in for Nelgin's null modem cable setup, but you can't (apparently) support both 8N1 and 7E1 "callers" with a real modem (not surprisingly, for multiple reasons).