with-connection-local-variables: Avoid code duplication

with-connection-local-variables: Avoid code duplication

Move the bulk of the code of `with-connection-local-variables` into
a separate function, which both avoids duplicating that code but also
avoids duplicating the code passed as the body of
a `with-connection-local-variables`.  Also makes it easier to
debug the code, or change the implementation of
`with-connection-local-variables` without having to recompile all
the users.

* lisp/files-x.el (with-connection-local-variables-1): New function,
extracted from `with-connection-local-variables`.
(with-connection-local-variables): Use it.