SI-8015 Count lines by EOLs

Source lines were counted by "line break chars", including FF.
Clients of `pos.line` seem to all expect the ordinary line num,
so that is what they get.

Unicode processing now precedes line ending processing.
SI-8015 Unprintables in messages

The line content to display should include the whole line,
inclusive of embedded line breaks, and any control chars
should be displayed in unicode escape.
SI-8015 Carat => Caret

Just a diamond in the rough.

Wikipedia warns against confusing it with "carrot."

The first word should really be pronounced as the garage phrase,
"Where's the car at?" and the second like "beret."
SI-8015 Refactor per code review

Make an obscure private def local and rename it usefully.

If you really must have similar functions in the API
(to detect line breaks and EOL), at least make them DRY.
Merge pull request #3285 from som-snytt/issue/8015-FF-NLs

 Count lines by EOLs