(sentence-end-double-space, sentence-end-without-period): Move to paragraphs.

(sentence-end-double-space, sentence-end-without-period): Move to paragraphs.
(fill-indent-according-to-mode): Change default to t.
(fill-context-prefix): Simplify control-flow and use a more
sophisticated merge that unifies both previous checks.
(fill-single-word-nobreak-p, fill-french-nobreak-p): New funs.
(fill-nobreak-predicate): Make it into a defcustom'd hook.
(fill-nobreak-p): New fun.
(fill-region-as-paragraph): Use it.
Handle `fill-indent-according-to-mode' slightly differently.
(fill-individual-paragraphs-prefix): Simplify the control-flow.
(fill-individual-paragraphs-citation): Fix.