Don't resume new threads if scheduler-locking is in effect

Don't resume new threads if scheduler-locking is in effect

If scheduler-locking is in effect, e.g., with "set scheduler-locking
on", and you step over a function that spawns a new thread, the new
thread is allowed to run free, at least until some event is hit, at
which point, whether the new thread is re-resumed depends on a number
of seemingly random factors.  E.g., if the target is all-stop, and the
parent thread hits a breakpoint, and GDB decides the breakpoint isn't
interesting to report to the user, then the parent thread is resumed,
but the new thread is left stopped.

I think that letting the new threads run with scheduler-locking
enabled is a defect.  This commit fixes that, making use of the new
clone events on Linux, and of target_thread_events() on targets where
new threads have no connection to the thread that spawned them.

Testcase and documentation changes included.

Approved-By: Eli Zaretskii <eliz@gnu.org>
Reviewed-By: Andrew Burgess <aburgess@redhat.com>
Change-Id: Ie12140138b37534b7fc1d904da34f0f174aa11ce