Three new configurable options:

Three new configurable options:
 - Robert de Bath's Compose key is now off by default and configurable on
 - The ages-old controversy over whether ALT by itself should bring the
   System menu up is now controllable by a config option
 - You can now independently configure whether scrollback resets on a
   keypress _and_ whether it resets on screen activity.

git-svn-id: svn://svn.tartarus.org/sgt/putty@741 cda61777-01e9-0310-a592-d414129be87e