bug#51051: Restore longlines.el to non-obsolete status

[Phil Sainty <psainty@orcon.net.nz>]

longlines.el was marked obsolete in 24.4 on the basis that
visual-line-mode provided a better alternative, and indeed
visual-line-mode is better for wrapping lines of normal lengths;
however longlines-mode is actually very valuable for editing
files with lines that are so long that they cause performance
problems.  This is because longlines-mode converts the long
lines into short lines for editing, and therefore it will give
you good performance at *any* position in the file.

For this reason, so-long.el has supported longlines-mode as a
standard action since version 1.0.  While so-long-mode is very
fast to enable, it still won't cope well if you move point deep
into an incredibly long line.  Conversely longlines-mode is slow
to enable (as it has to break all the long lines), but it then
gives good performance everywhere in the buffer.  As such they
complement one another (and you can switch between them via the
"So Long" menu).

As longlines.el is still valuable for reasons unrelated to
visual-line-mode, I'd like it to be restored to non-obsolete
status.  I think the relationship with visual-line-mode (and
recommendation of the latter for line-wrapping in general) can
just be documented in the longlines commentary.


-Phil