The `wrapnext' variable now states whether we _would_ wrap next

The `wrapnext' variable now states whether we _would_ wrap next
character if we were wrapping, not whether we _will_ wrap next
character. Makes for saner behaviour with vertical-line cursor and
also when changing autowrap mode while on rightmost column. Does
entail small behavioural changes to backspace and destructive-
backspace when in rightmost column with Auto Wrap off, but I don't
think they should be catastrophic, or indeed that there's a well
defined Right Behaviour.

git-svn-id: svn://svn.tartarus.org/sgt/putty@872 cda61777-01e9-0310-a592-d414129be87e