codec/line: do charset conversion on flush

Add tests to verify line codec does charset conversion on flush.

Fixes #1520
Remove the "how to get help" banner because nobody reads it.

There's several indications that folks don't actually read this banner
when unexpected errors occur. If nobody's reading it, then it's just
wasting space in the output.

I would cite references, but will not because there's no need to shame
anyone.

Fixes #1442