* tramp.el (tramp-root-regexp): New defconst.

* tramp.el (tramp-root-regexp): New defconst.
(tramp-completion-file-name-regexp-unified)
(tramp-completion-file-name-regexp-separate)
(tramp-completion-file-name-regexp-url): Use it.
(tramp-do-copy-or-rename-file-via-buffer): Set
`enable-multibyte-characters' to nil.  Set `jka-compr-inhibit' to
t for `insert-file-contents-literally'.
(tramp-drop-volume-letter): Rewrite, using `tramp-root-regexp'.
Autoload it.
(tramp-completion-file-name-handler-post-function): New defconst.
(tramp-completion-file-name-handler): Use it.
(tramp-maybe-open-connection): Update calls to
`tramp-flush-connection-property' for removed 2nd argument.