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.

lib/fatal-signal.c

@@ -134,6 +134,17 @@ fatal_signal_handler(int sig_nr)
     stored_sig_nr = sig_nr;
 }
 
+/* Check whether a fatal signal has occurred and, if so, call the fatal signal
+ * hooks and exit.
+ *
+ * This function is called automatically by poll_block(), but specialized
+ * programs that may not always call poll_block() on a regular basis should
+ * also call it periodically.  (Therefore, any function with "block" in its
+ * name should call fatal_signal_run() each time it is called, either directly
+ * or through poll_block(), because such functions can only used by specialized
+ * programs that can afford to block outside their main loop around
+ * poll_block().)
+ */
 void
 fatal_signal_run(void)
 {