Improve symlinks for Tramp

Improve symlinks for Tramp

* lisp/files.el (files--splice-dirname-file): Quote whole file.

* lisp/net/tramp-sh.el (tramp-sh-handle-make-symbolic-link):
Do not expand TARGET, it could be remote.
(tramp-sh-handle-file-truename): Check for cyclic symlink also
in case of readlink.  Quote result if it looks remote.
(tramp-sh-handle-file-local-copy): Use `file-truename'.

* test/lisp/net/tramp-tests.el (tramp-test08-file-local-copy)
(tramp-test09-insert-file-contents): Test also file missing.
(tramp-test21-file-links): Extend test.