A couple of X forwarding fixes for Unix Plink. Firstly, under Unix

A couple of X forwarding fixes for Unix Plink. Firstly, under Unix
the default X display should be whatever comes out of $DISPLAY,
rather than Windows's hardwired `localhost:0'. Secondly, this may
give rise to a display name without a hostname (`:0' or similar),
which we now need to be able to deal with. Of course, we still don't
_properly_ support X forwarding in Unix Plink, since we still can't
authenticate with the local display.

[originally from svn r2420]