Unlock dosigsuspend() and with that some aspects of ppoll and pselect

Unlock dosigsuspend() and with that some aspects of ppoll and pselect

Change p_sigmask from atomic back to non-atomic updates. All changes to
p_sigmask are only allowed by curproc (the owner). There is no need for
atomic instructions here.

p_sigmask is mostly accessed by curproc with the exception of ptsignal().
In ptsignal() p_sigmask is now only read once unless a SSLEEP proc gets
the signal. In that case recheck the p_sigmask before wakeup to ensure
that no unnecessary wakeup happens.

Add some KASSERT(p == curproc) to ensure this precondition.
sigabort() is special since it is also called by ddb but apart from that
only works for curproc.

With and OK mvs@ OK mpi@