Merge changes from Tramp repository

Merge changes from Tramp repository

* doc/misc/Makefile.in (${buildinfodir}/tramp.info tramp.html):
No EXTRA_OPTS needed.

* doc/misc/tramp.texi: Merge changes from Emacsemacs-25
branch, especially for @trampfn{}.
(Top): Move @ifnottex down.
(History): XEmacs support has been removed.
(GVFS based methods, Remote processes): Do not use emacsgvfs flag.
(Auto-save and Backup): Use both syntax versions.
(File name Syntax): Remark on IPv6 adresses is valid for
unified syntax only.

* doc/misc/trampver.texi: Do not set emacsgvfs flag.