Sync with Tramp 2.2.6-pre.

Sync with Tramp 2.2.6-pre.

* net/tramp.el (tramp-drop-volume-letter): Provide an XEmacs
compatible declaration.

* net/tramp-cmds.el (tramp-append-tramp-buffers): Protect
`list-load-path-shadows' call.

* net/tramp-compat.el (top): Require packages, which aren't
autoloaded anymore for XEmacs.  Protect call of
`tramp-file-name-handler' by `tramp-compat-funcall', pacifying the
compiler.  Do not require tramp-util.el and tramp-vc.el anymore,
it hurts at least for SXEmacs.
(tramp-compat-temporary-file-directory): In XEmacs, there is no
standard-value for `temporary-file-directory'.

* net/tramp-sh.el (tramp-do-directory-files-and-attributes-with-stat):
Redirect stderr to /dev/null.
(tramp-sh-handle-write-region): uid and gid can be floats.
Reported by Russell Sim <russell.sim@gmail.com>.
(tramp-sh-handle-vc-registered): Hide errors.
(tramp-vc-file-name-handler): Use dummy results for `process-file'
and `start-file-process'.
(tramp-maybe-open-connection): Check also whether `non-essential'
is bound.