infrun: scheduler-locking replay

infrun: scheduler-locking replay

Record targets behave as if scheduler-locking were on in replay mode.  Add a
new scheduler-locking option "replay" to make this implicit behaviour explicit.
It behaves like "on" in replay mode and like "off" in record mode.

By making the current behaviour a scheduler-locking option, we allow the user
to change it.  Since it is the current behaviour, this new option is also
the new default.

One caveat is that when resuming a thread that is at the end of its execution
history, record btrace implicitly stops replaying other threads and resumes
the entire process.  This is a convenience feature to not require the user
to explicitly move all other threads to the end of their execution histories
before being able to resume the process.

We mimick this behaviour with scheduler-locking replay and move it from
record-btrace into infrun.  With all-stop on top of non-stop, we can't do
this in record-btrace anymore.

Record full does not really support multi-threading and is therefore not
impacted.  If it were extended to support multi-threading, it would 'benefit'
from this change.  The good thing is that all record targets will behave the
same with respect to scheduler-locking.

I put the code for this into clear_proceed_status.  It also sends the
about_to_proceed notification.

gdb/
	* NEWS: Announce new scheduler-locking mode.
	* infrun.c (schedlock_replay): New.
	(scheduler_enums): Add schedlock_replay.
	(scheduler_mode): Change default to schedlock_replay.
	(user_visible_resume_ptid): Handle schedlock_replay.
	(clear_proceed_status_thread): Stop replaying if resumed thread is
	not replaying.
	(schedlock_applies): Handle schedlock_replay.
	(_initialize_infrun): Document new scheduler-locking mode.
	* record-btrace.c (record_btrace_resume): Remove code to stop other
	threads when not replaying the resumed thread.

doc/
	* gdb.texinfo (All-Stop Mode): Describe new scheduler-locking mode.