Prefer defvar-local in nxml/*.el

Prefer defvar-local in nxml/*.el

* lisp/nxml/rng-cmpct.el (rng-c-current-token)
(rng-c-escape-positions, rng-c-file-name):
* lisp/nxml/rng-pttrn.el (rng-current-schema):
* lisp/nxml/rng-valid.el (rng-validate-timer)
(rng-validate-quick-timer, rng-error-count, rng-message-overlay)
(rng-message-overlay-inhibit-point, rng-message-overlay-current)
(rng-validate-up-to-date-end, rng-conditional-up-to-date-start)
(rng-conditional-up-to-date-end, rng-dtd): Prefer defvar-local.