This changes do_sigaction to avoid read_lock(&tasklist_lock) on every
call.  Only in the fairly uncommon cases where it's really needed will
it take that lock (which requires unlocking and relocking the siglock
for locking order).

I also changed the ERESTARTSYS added in my earlier patch to ERESTARTNOINTR.
That is an "instantaneous" case, and there is no reason to have it possibly
return EINTR if !SA_RESTART (which AFAIK sigaction never could before, and
it might not be kosher by POSIX); rollback is always better.