Prepare to have multiple X11 auth cookies valid at once.

Prepare to have multiple X11 auth cookies valid at once.

Rather than the top-level component of X forwarding being an
X11Display structure which owns some auth data, it's now a collection
of X11FakeAuth structures, each of which owns a display. The idea is
that when we receive an X connection, we wait to see which of our
available auth cookies it matches, and then connect to whatever X
display that auth cookie identifies. At present the tree will only
have one thing in it; this is all groundwork for later changes.

git-svn-id: http://svn.tartarus.org/sgt/putty@10079 cda61777-01e9-0310-a592-d414129be87e