Added support for a new "user event": local/sysop chat

If an external progarm is configured (in SCFG->External Progarms) is configured
to "execute on event" set to "Local/Sysop Chat", then that program will be
executed when the remote user is "pulled" into sysop-chat (e.g. via F10 key in
sbbsctrl) rather than running the normal built-in chat.
Don't change the current node's action (to XTRN) if it's currently PCHT
(private chat) as that's a signal to the local sysop chat app that the user
has disconnected.