xwayland: Add local user to xhost
With the addition of xauth support (commit a8984a81), Xwayland would rely only on the provided cookies for authentication.
As a result, running an Xclient from another VT (hence without the XAUTHORITY environment variable set) would result in an access denied.
The same on X11 is granted because the local user is automatically granted access to Xserver by the startup scripts.
Add the local user to xhost at startup on Xwayland so that the user can still run a client by setting the DISPLAY as long as it's the same user on the same host.