core: do not fail in a container if we can't use setgroups

It might be blocked through /proc/PID/setgroups

src/basic/user-util.h

@@ -86,3 +86,5 @@ bool valid_user_group_name(const char *u);
 bool valid_user_group_name_or_id(const char *u);
 bool valid_gecos(const char *d);
 bool valid_home(const char *p);
+
+int maybe_setgroups(size_t size, const gid_t *list);