Robert de Bath's multi-purpose patch, slightly modified.

Robert de Bath's multi-purpose patch, slightly modified.
 - ^E answerback is now `PuTTY'.
 - The framework is now in place for the scrollback to reset to
   bottom on display _or_ keyboard events _or_ both. An actual
   configurable option isn't yet present, but most of the code is in
   place.
 - Try to deal with the problems where incoming data gets dropped
   after decoding but before display.
 - Scrollback behaviour has changed: instead of keying it off
   `scroll' versus `delete top line', things now go into the
   scrollback from _either_ of those but only if the primary screen
   is selected. Should fix problems with `less' and talkers.
 - must_update variable has gone because rdb correctly observed that
   it didn't seem to be doing a great deal :-)

[originally from svn r328]