Code review comment for lp:~vila/bzr/353370-notty-no-term-width

Since there are no single answer valid for all cases, I went for two different ones:
- one for the tty case 80,
- one for the non-tty case 256.

The COLUMNS environment variable is always respected *first* so that users have a way
to override any heuristic we may use.

pagers can use either the COLUMNS variable or go for the bzrlib.osutils default values
(+default_tty_width or +default_non_tty_width).

I added tests for most of the cases except windows (since I can't test that), if any reviewer can
add a such a test I'll gladly add it.

I also addressed bug #62539 while I was there.