Allow pg_ctl kill to send SIGKILL.

Previously that was disallowed out of an abundance of
caution. Providing KILL support however is helpful to make the
013_crash_restart.pl test portable, and there's no actual issue with
allowing it.  SIGABRT, which has similar consequences except it also
dumps core, was already allowed.

Author: Andres Freund
Discussion: https://postgr.es/m/45d42d41-6145-9be1-7261-84acf6d9e344@2ndQuadrant.com

src/bin/pg_ctl/pg_ctl.c

@@ -1903,7 +1903,7 @@ do_help(void)
 	printf(_("  immediate   quit without complete shutdown; will lead to recovery on restart\n"));
 
 	printf(_("\nAllowed signal names for kill:\n"));
-	printf("  ABRT HUP INT QUIT TERM USR1 USR2\n");
+	printf("  ABRT HUP INT KILL QUIT TERM USR1 USR2\n");
 
 #ifdef WIN32
 	printf(_("\nOptions for register and unregister:\n"));
@@ -1961,11 +1961,8 @@ set_sig(char *signame)
 		sig = SIGQUIT;
 	else if (strcmp(signame, "ABRT") == 0)
 		sig = SIGABRT;
-#if 0
-	/* probably should NOT provide SIGKILL */
 	else if (strcmp(signame, "KILL") == 0)
 		sig = SIGKILL;
-#endif
 	else if (strcmp(signame, "TERM") == 0)
 		sig = SIGTERM;
 	else if (strcmp(signame, "USR1") == 0)