x11/xdm: Use $LOCALBASE/etc/xdm/Xservers as base for Xserver args

Hard coding Xserver args forces people to invoke xdm through init
by placing an entry in /etc/ttys when Xserver args different from
those hardcoded in rc.d/xdm are desired. Instead glean arguments
from $LOCALBASE/etc/xdm/Xservers, defaulting to the hardcoded
entry if none are found for :0.

Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D42097