BUG/MINOR: checks: successful check completion must not re-enable MAINT servers

If a server is switched to maintenance mode while a check is in progress,
the successful completion of the check must not switch it back up. This
is still a consequence of using the same function set_server_up() for
every state change. Bug reported by Igor at owind.

This fix should be backported to 1.4 which is affected as well.
diff --git a/src/checks.c b/src/checks.c
index 115cc85..bdb8f07 100644
--- a/src/checks.c
+++ b/src/checks.c
@@ -1682,7 +1682,8 @@
 					set_server_disabled(check);
 			}
 
-			if (check->health < check->rise + check->fall - 1) {
+			if (!(s->state & SRV_MAINTAIN) &&
+			    check->health < check->rise + check->fall - 1) {
 				check->health++; /* was bad, stays for a while */
 				set_server_up(check);
 			}