sigfastblock: fix delivery of the pending signals in single-threaded processes.

If single-threaded process receives a signal during critical section
established by sigfastblock(2) word, unblock did not caused signal
delivery because sigfastblock(SIGFASTBLOCK_UNBLOCK) failed to request
ast handling of the pending signals.

Set TDF_ASTPENDING | TDF_NEEDSIGCHK on unblock or when kernel forces
end of sigfastblock critical section, to cause syscall exit to recheck
and deliver any signal pending.

Reported by: corydoras@ridiculousfish.com
PR: 246385
Sponsored by: The FreeBSD Foundation