ptyfwd: tweaks to background tinting logic

ptyfwd: tweaks to background tinting logic

This makes two changes:

1. it resets the background color not only on NL (aka LF) but also on
   CR, but without erasing things to the end of the line. This increases
   compatbility with tools such as "less" which use CR to jump back to
   the beginning of the line.

2. previously we'd not process series of newlines or ansi sequences
   without intermediate other characters correctly, we'd always assume
   what follows is regular text. Fix that, and correctly determine the
   right state from the subsequent character.