3 new configurable system loadable modules:

- nodelist (default: "nodelist")
- who's online (default: "nodelist -active")
- private message (default: "privatemsg")

This allows the nodelist and private message modules to be "installed" without
over-riding global hot key events (SCFG->External Programs->Global Hot Key
Events), allows the Baja functions NODELIST_ALL, NODELIST_USERS, and
PRIVATE_MESSAGE and JS bbs.list_nodes(), bbs.whos_online(), and
bbs.private_message() methods to use external modules (JS or Baja)
automatically.

The old C++ logic remains for these features if the module's are cleared (set
to a blank string) in SCFG, but at some point, that code will likely be removed.