Don't unconditionally reset scrollback on certain escape sequences.

Don't unconditionally reset scrollback on certain escape sequences.

Handlers for a number of escape sequences, notably including ESC[J and
the sequences that switch to/from the alternate screen, were
unconditionally resetting the scrollback instead of first checking the
'Reset scrollback on display activity' configuration option. I've
added the missing if statements, so now 'Reset scrollback on display
activity' should actually mean what it says.

For example, this would have inconvenienced an mplayer user, who
wouldn't be able to go up and check their scrollback while mplayer was
repeatedly redisplaying its status line, because mplayer uses ESC[J to
erase each version of the status line before printing the next
version.

git-svn-id: http://svn.tartarus.org/sgt/putty@10125 cda61777-01e9-0310-a592-d414129be87e