- Merge struct procsig with struct sigacts.

- Merge struct procsig with struct sigacts.
- Move struct sigacts out of the u-area and malloc() it using the
  M_SUBPROC malloc bucket.
- Add a small sigacts_*() API for managing sigacts structures: sigacts_alloc(),
  sigacts_free(), sigacts_copy(), sigacts_share(), and sigacts_shared().
- Remove the p_sigignore, p_sigacts, and p_sigcatch macros.
- Add a mutex to struct sigacts that protects all the members of the struct.
- Add sigacts locking.
- Remove Giant from nosys(), kill(), killpg(), and kern_sigaction() now
  that sigacts is locked.
- Several in-kernel functions such as psignal(), tdsignal(), trapsignal(),
  and thread_stopped() are now MP safe.

Reviewed by:	arch@
Approved by:	re (rwatson)