Get rid of the error-return mechanism from x11_init.

Get rid of the error-return mechanism from x11_init.

Now that it doesn't actually make a network connection because that's
deferred until after the X authorisation exchange, there's no point in
having it return an error message and write the real output through a
pointer argument. Instead, we can just have it return xconn directly
and simplify the call sites.

git-svn-id: http://svn.tartarus.org/sgt/putty@10081 cda61777-01e9-0310-a592-d414129be87e