Add helpers to dynamically assign connection-local values

Add helpers to dynamically assign connection-local values

* lisp/files-x.el (connection-local-criteria)
(connection-local-profile-name-for-setq): New variables.
(with-connection-local-variables-1): ... let-bind them here.
(connection-local-update-profile-variables)
(connection-local-profile-name-for-criteria): New functions.
(with-connection-local-application-variables, setq-connection-local):
New macros.

* test/lisp/files-x-tests.el: Require 'tramp-integration'
(files-x-test--variable5, remote-lazy-var): New variables.
(files-x-test-hack-connection-local-variables-apply): Expand checks.
(files-x-test-with-connection-local-variables): Remove
'hack-connection-local-variables-apply' check (it belongs in the above
test), and expand some other checks.
(files-x-test--get-lazy-var, files-x-test--set-lazy-var): New
functions.
(files-x-test-connection-local-update-profile-variables)
(files-x-test-setq-connection-local): New tests.

* doc/lispref/variables.texi (Connection Local Variables): Split into
two subsections and document the new features.

* etc/NEWS: Announce 'setq-connection-local'.