* infrun.c (context_switch): New function. Abstract the operation
of saving and restoring infrun's state when switching threads.
(handle_inferior_event): Normalize the handling of the 'thread hop'
event (when the wrong thread hits a thread-specific breakpoint,
and we need to solo-step that thread past the breakpoint).
Call keep_going, instead of target_resume. Handle the subsequent
singlestep-trap as a normal event instead of just resuming.