Implement Richard's really clever idea about bell overload mode:

Implement Richard's really clever idea about bell overload mode:
it's automatically deactivated by any keypress, so that command-line
beeps from (e.g.) filename completion don't suddenly stop occurring,
but it still provides a rapid response to an accidental spewing of a
binary to your terminal.

git-svn-id: svn://svn.tartarus.org/sgt/putty@2107 cda61777-01e9-0310-a592-d414129be87e