Rework how action SIG_HOLD is handled in ptsignal.

Rework how action SIG_HOLD is handled in ptsignal.

Since we want to unlock sigsuspend, ptsignal needs to double check in the
SSLEEP case that the signal being delivered is still masked or unmasked.
Remove the early return for action SIG_HOLD so that the SSLEEP case can
properly recheck the sigmask.

On top of this update siglist only in one place at the end of ptsignal
this now includes the clearing of signals for the SA_CONT and SA_STOP
cases.

OK mpi@