core: map the 'rescue' argument to rescue.target

Even though the 'emergency' and 'single' aliases come from sysvinit, the
lack of 'rescue' is still quite confusing (caught me by surprise for the
9th time yet) and inconsistent with `systemctl rescue` as well.

src/core/main.c

@@ -272,6 +272,7 @@ static int parse_proc_cmdline_item(const char *key, const char *value) {
         static const char * const rlmap[] = {
                 "emergency", SPECIAL_EMERGENCY_TARGET,
                 "-b",        SPECIAL_EMERGENCY_TARGET,
+                "rescue",    SPECIAL_RESCUE_TARGET,
                 "single",    SPECIAL_RESCUE_TARGET,
                 "-s",        SPECIAL_RESCUE_TARGET,
                 "s",         SPECIAL_RESCUE_TARGET,