* net/tramp.el (top): Require cl.el, when `copy-tree' is not available

* net/tramp.el (top): Require cl.el, when `copy-tree' is not available
otherwise.
(tramp-get-remote-path): New defun.  Replace occurences of
`tramp-default-remote-path' by this function.
(tramp-set-remote-path): Move most of the code to
`tramp-get-remote-path'.
(tramp-get-ls-command, tramp-get-remote-id): Don't check for not
existing directories, this is done already in
`tramp-get-remote-path'.