Use the process ps_mtx to protect the process sigacts structure.

Use the process ps_mtx to protect the process sigacts structure.
With this cursig(), postsig() and trapsignal() become safe to be called
without KERNEL_LOCK. As a side-effect sleep with PCATCH no longer needs
the KERNEL_LOCK either. Since sending a signal can happen from interrupt
context raise the ps_mtx IPL to high.
Feedback from mpi@ and kettenis@
OK kettenis@