Fix signal delivery might be on sigfastblock clearing.

When clearing sigfastblock, either by sigfastblock(UNSETPTR) call or
implicitly on execve(2), kernel must check for pending signals and
reschedule them if needed.

E.g. on execve, all other threads are terminated, and current thread
fast block pointer is cleaned. If any signal was left pending, it can
now be delivered to the current thread, and we should prepare for
ast() on return to userspace to notice the signals.

Reported and tested by: pho
Sponsored by: The FreeBSD Foundation