Fix for spurious Space getting sent when alt_space and alt_only are

Fix for spurious Space getting sent when alt_space and alt_only are
both set and you bring up the Sysmenu with an alt_space and dispatch
it with an alt_only. (The SYSKEYDOWN for alt_only is never received,
but we get the SYSKEYUP which PostMessages the space since it
expects to be triggering the _creation_ of a sysmenu. Solution: set
alt_state to 0 when an alt_space triggers a sysmenu, so that the
final SYSKEYUP will be seen as spurious, which it is. Perhaps we
could do this better.)

git-svn-id: svn://svn.tartarus.org/sgt/putty@848 cda61777-01e9-0310-a592-d414129be87e