* lisp/tramp-gvfs.el (tramp-gvfs-methods) <sftp>: Add.

* lisp/tramp-sh.el (tramp-methods) <sftp>: Remove.  It has never worked
satisfactorily.
(top): Remove completion function for "sftp".
* texi/tramp.texi (External methods): Remove `sftp'.
(GVFS based methods): Add `sftp'.
* tramp-gvfs.el (tramp-gvfs-handle-file-attributes): Use `string-to-number'
when reading "unix::mode".
* tramp-gvfs.el (tramp-gvfs-url-file-name): `user' and `localname' could be nil.