Truncate all terminal lines when we clear scrollback.

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.

[originally from svn r10210]