Move SCHED_LOCK after sleep_signal_check.

Move SCHED_LOCK after sleep_signal_check.

sleep_signal_check() is there to look for pending signals / single thread
requests which were posted before sleep_setup() finished. Once p_stat
is set to SSLEEP the wakeup and delivery of signals is taken care of
by ptsignal and single_thread_set().

Moving the SCHED_LOCK further down allows to cleanup cursig() and to
remove a SCHED_LOCK recursion in single_thread_check().

OK mpi@