gdb/infrun: add inferior parameters to stop_all_threads and restart_threads

gdb/infrun: add inferior parameters to stop_all_threads and restart_threads

A following patch will want to stop all threads of a given inferior (as
opposed to all threads of all inferiors) while handling a vfork, and
restart them after.  To help with this, add inferior parameters to
stop_all_threads and restart_threads.  This is done as a separate patch
to make sure this doesn't cause regressions on its own, and to keep the
following patches more concise.

No visible changes are expected here, since all calls sites pass
nullptr, which should keep the existing behavior.

Change-Id: I4d9ba886ce842042075b4e346cfa64bbe2580dbf