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.

[originally from svn r10081]