systemctl: fix return code in chroot

If 'systemctl enable' (and friends) is run inside chroot it always
exits with a bad return code. unit_file_enable() returns the number of
symlink rules that were supposed to be created. So resetting r to 0 and
exiting gracefully should be the correct way.

src/systemctl/systemctl.c

@@ -3630,6 +3630,7 @@ static int enable_unit(DBusConnection *bus, char **args) {
                         }
                 }
 
+                r = 0;
         } else {
                 const char *method;
                 bool send_force = true, expect_carries_install_info = false;