Sync with Tramp 2.2.6-pre.

Sync with Tramp 2.2.6-pre.

* tramp.texi (all): Use consequently @command{}, @env{} and @kbd{}
where appropriate.
(Ad-hoc multi-hops): New section.
(Remote processes): New subsection "Running remote processes on
Windows hosts".
(History): Add remote commands on Windows, and ad-hoc multi-hop methods.
(External methods): "ControlPersist" must be set to "no" for the
`scpc' method.
(Remote processes): Add a note about `auto-revert-tail-mode'.
(Frequently Asked Questions): Use "scpx" in combination with
"ControlPersist".  Reported by Adam Spiers <emacs@adamspiers.org>.

* trampver.texi: Update release number.