Truncate all terminal lines when we clear scrollback.

Now Jacob has reminded me that 'resize-no-truncate' was already on the
wishlist, I notice that it suggested Clear Scrollback should remove
the preserved information off to the right. On the basis that that's
(at least partly) a privacy feature, that seems sensible, so let's do it.


git-svn-id: svn://svn.tartarus.org/sgt/putty@10210 cda61777-01e9-0310-a592-d414129be87e