namespace-util: add helper to get base UID from userns

src/basic/namespace-util.h

@@ -91,3 +91,5 @@ int netns_acquire(void);
 int parse_userns_uid_range(const char *s, uid_t *ret_uid_shift, uid_t *ret_uid_range);
 
 int is_idmapping_supported(const char *path);
+
+int userns_get_base_uid(int userns_fd, uid_t *ret_uid, gid_t *ret_gid);