update credentials when reloading a service

Many long-running services will reload configuration and therefore
credentials on reload. Therefore its useful if a service reload will
also update credentials.

Tested in https://github.com/numtide/systemd-vaultd/pull/12

src/core/service.c

@@ -2422,7 +2422,7 @@ static void service_enter_reload(Service *s) {
                 r = service_spawn(s,
                                   s->control_command,
                                   s->timeout_start_usec,
-                                  EXEC_APPLY_SANDBOXING|EXEC_APPLY_CHROOT|EXEC_IS_CONTROL|EXEC_CONTROL_CGROUP,
+                                  EXEC_APPLY_SANDBOXING|EXEC_APPLY_CHROOT|EXEC_IS_CONTROL|EXEC_CONTROL_CGROUP|EXEC_WRITE_CREDENTIALS,
                                   &s->control_pid);
                 if (r < 0)
                         goto fail;