* lisp/hi-lock.el: Refine the choice of default face.

* lisp/hi-lock.el: Refine the choice of default face.
(hi-lock-keyword->face): New function.  Use it wherever we used
cadadadr instead.
(hi-lock--regexps-at-point): Ignore faces that can't come from hi-lock.
(hi-lock--last-face): Remove var.
(hi-lock--unused-faces): New var to replace it.
(hi-lock-read-face-name): Use/maintain it.
(hi-lock-unface-buffer): Maintain it.  Fix error for the C-u case.
(hi-lock-set-pattern): Ignore new rule if it has the same regexp even
if it has another face.