Implement connection-local variables

Implement connection-local variables

* doc/lispref/variables.texi (Connection Local Variables): New section.

* etc/NEWS: Mention connection-local variables.

* lisp/files-x.el (enable-connection-local-variables)
(connection-local-variables-alist, connection-local-class-alist)
(connection-local-criteria-alist): New defvars.
(connection-local-get-classes)
(connection-local-get-class-variables): New defsubst.
(connection-local-set-classes)
(connection-local-set-class-variables)
(hack-connection-local-variables)
(hack-connection-local-variables-apply): New defuns.
(with-connection-local-classes): New defmacro.

* lisp/net/tramp.el (tramp-set-connection-local-variables): New defun.

* lisp/net/tramp-adb.el (tramp-adb-maybe-open-connection):
* lisp/net/tramp-gvfs.el (tramp-gvfs-maybe-open-connection):
* lisp/net/lisp/net/tramp-sh.el (tramp-maybe-open-connection):
* lisp/net/tramp-smb.el (tramp-smb-maybe-open-connection): Use it.

* test/lisp/files-x-tests.el: New file.