bug#14973: Unfill for Emacs

[Drew Adams <drew.adams@oracle.com>]

Just one terminology point: what you describe and implement is not
unfilling.  These are fill commands, not unfill commands.

Unfilling is the inverse of filling.  Unfilling undoes the last filling
operation on the same region or whatever.  It restores contiguous
whitespace chars that were collapsed, etc.

What you have instead might be called "join lines" or "remove line
breaks".  But even those names do not accurately characterize it, because
it also fills: it insists on the kind of whitespace separation imposed
by filling: no contiguous whitespace chars (except possibly indentation
and after sentence ends).

That does not mean that this could not be useful.  But we might not want
to falsely advertise what it does.

The best characterization of what this code does is "fill as one line".
(But even that assumes a line length less than `most-positive-fixnum'.)
And that is my suggestion:

 `fill-paragraph-as-one-line'
 `fill-region-as-one-line'