(fill-region-as-paragraph): Don't find adaptive-fill-regexp

(fill-region-as-paragraph): Don't find adaptive-fill-regexp
on first line of paragraph if it's a paragraph-separate line.
Don't look past the intended line.
(adaptive-fill-function): New variable.
(fill-region-as-paragraph): Use it.
(colon-double-space): New variable.
(canonically-space-region): Put two spaces after colon if necessary.