(fill-context-prefix): If we get a prefix from the

(fill-context-prefix): If we get a prefix from the
second line of the paragraph, verify the first line has it too.

(adaptive-fill-regexp): Allow - and |;
allow spaces within the prefix.  Allow numeric headings.
(adaptive-fill-first-line-regexp): New variable.
(fill-context-prefix): Use adaptive-fill-first-line-regexp.
If prefix from first line seems unreasonable, use whitespace instead.
Reject it entirely only if it is a paragraph-starter.