Some microoptimizations.

MFC after:	1 month
Fix SA_SIGINFO | SA_RESETHAND handling. The sysent' sv_sendsig()
method needs pre-reset state of the ps_siginfo to correctly construct
signal frame.

Move sigdflt() call after the sv_sendsig() invocation in postsig().
Simultaneously extract common code from trapsignal() and postsig()
into new helper postsig_done().

Submitted by:	rea
MFC after:	1 week
The process spin lock currently has the following distinct uses:

- Threads lifetime cycle, in particular, counting of the threads in
  the process, and interlocking with process mutex and thread lock.
  The main reason of this is that turnstile locks are after thread
  locks, so you e.g. cannot unlock blockable mutex (think process
  mutex) while owning thread lock.

- Virtual and profiling itimers, since the timers activation is done
  from the clock interrupt context.  Replace the p_slock by p_itimmtx
  and PROC_ITIMLOCK().

- Profiling code (profil(2)), for similar reason.  Replace the p_slock
  by p_profmtx and PROC_PROFLOCK().

- Resource usage accounting.  Need for the spinlock there is subtle,
  my understanding is that spinlock blocks context switching for the
  current thread, which prevents td_runtime and similar fields from
  changing (updates are done at the mi_switch()).  Replace the p_slock
  by p_statmtx and PROC_STATLOCK().

The split is done mostly for code clarity, and should not affect
scalability.

Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week