fbsd-nat: Add a list of pending events.

fbsd-nat: Add a list of pending events.

The m_pending_events list stores a queue of deferred events that might
be reported by the next call to the target's wait method.  The set of
events that are eligible is filtered by the ptid passed to resume.

For now this just replaces the list of vfork_done events.  A
subsequent commit will reuse this to store other events.