nspawn: when readjusting UID/GID ownership of OS trees, skip read-only subtrees

This should allow tools like rkt to pre-mount read-only subtrees in the OS
tree, without breaking the patching code.

Note that the code will still fail, if the top-level directory is already
read-only.

src/basic/fd-util.h

@@ -72,6 +72,8 @@ void cmsg_close_all(struct msghdr *mh);
 
 bool fdname_is_valid(const char *s);
 
+int fd_get_path(int fd, char **ret);
+
 /* Hint: ENETUNREACH happens if we try to connect to "non-existing" special IP addresses, such as ::5 */
 #define ERRNO_IS_DISCONNECT(r) \
         IN_SET(r, ENOTCONN, ECONNRESET, ECONNREFUSED, ECONNABORTED, EPIPE, ENETUNREACH)