Move the (require 'cl) to the front of the

Move the (require 'cl) to the front of the
file where it's more visible.
(texinfo-filter, texinfo-chapter-level-regexp): New variables
moved from texnfo-upd.el (for AUCTeX's compatibility, originally).
(texinfo-mode): Use texinfo-chapter-level-regexp for page-delimiter.
(texinfo-inside-macro-p): Only catch `scan-error's.
(texinfo-inside-env-p): Make better use of the match info.
(texinfo-insert-quote): Collapse calls to `texinfo-inside-macro-p'.
(texinfo-insert-@end): Slight re-organization.
Also remove useless `looking-at' call.