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

[cy]

Summary

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.

Test Plan

Tested and working here.

[cy]

The original patch didn't take into account that a user might change the DisplayManager.servers resource to point to different file from the default file. The DisplayManager.servers resource could also be set to a string (just like -servers sets the resource), meaning it may not be a file. xdm will assume it's a file if the string begins with a slash ("/"). We need to take this also into account.

Again, this has been tested and is currently running here.