x11/xdm: allow X server arguments to be set in Xservers

This patch changes the rc.d script so that:

1- The xdm_tty variable is set to 'default'. This disable the override
and allows xdm to be configured the normal way. xdm uses the ttyv8
terminal by default anyway.

2- The server arguments are only overridden if the user explicitly
specify a value for xdm_tty.

PR: 265578