fd.o#18832: tp_get_bus: Document that this can call exit(), and why
fd.o#18832: tp_get_bus: Document that this can call exit(), and why

Also factor out starter_bus_conn() (actually caching the starter
bus) into a helper function.

This partially addresses fd.o#18832, which complains that tp-glib
calls exit() (which we can't remove until we break ABI).

Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib