Make fatal signals cause an exit more promptly in special cases.

Make fatal signals cause an exit more promptly in special cases.

The fatal-signal library notices and records fatal signals (e.g. SIGTERM)
and terminates the process on the next trip through poll_block().  But
some special utilities do not always invoke poll_block() promptly, e.g.
"ovs-ofctl monitor" does not call poll_block() as long as OpenFlow messages
are available.  But these special cases seem like they are all likely to
call into functions that themselves block (those with "_block" in their
names).  So make a new rule that such functions should always call
fatal_signal_run(), either directly or through poll_block().  This commit
implements and documents that rule.

Bug #2625.