Convert SCHED_LOCK from a recursive kernel lock to a mutex.

Convert SCHED_LOCK from a recursive kernel lock to a mutex.

Over the last weeks the last SCHED_LOCK recursion was removed so this
is now possible and will allow to split up the SCHED_LOCK in a upcoming
step.

Instead of implementing an MP and SP version of SCHED_LOCK this just
always uses the mutex implementation.
While this makes the local s argument unused (the spl is now tracked by
the mutex itself) it is still there to keep this diff minimal.

Tested by many.
OK jca@ mpi@