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, like 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>
Change-Id: Ie12140138b37534b7fc1d904da34f0f174aa11ce