Jacob's patch to make full-screen mode available even when Alt-Enter

Jacob's patch to make full-screen mode available even when Alt-Enter
isn't configured to do it: we now have a sysmenu option for it. In
addition, there's a special case such that clicking the top left
pixel on the screen in full-screen mode brings up the sysmenu (so
you can still get at the sysmenu in FS mode without Alt-anything
configured to bring it up and without ghastly fiddling with the
Windows key). Also Change Settings while in full-screen mode now
adds/removes the scrollbar in response to scrollbar_in_fullscreen
rather than scrollbar (oops).

git-svn-id: svn://svn.tartarus.org/sgt/putty@1410 cda61777-01e9-0310-a592-d414129be87e