Fix highlight-regexp's use of Font Lock mode.

Fix highlight-regexp's use of Font Lock mode.

* hi-lock.el (hi-lock-mode): Do not unilaterally enable font lock.
(hi-lock-set-pattern): When deciding whether to use font lock or
overlays, look at font-lock-mode instead of font-lock-fontified.
(hi-lock-mode, hi-lock-line-face-buffer, hi-lock-unface-buffer)
(hi-lock-face-buffer, hi-lock-face-phrase-buffer): Doc fix.

Fixes: debbugs:12168