Allow mod namespace for door registrations.

This is an adapted version of #861 - by oleastre

Most mods had been calling `doors.register_door() with a door
name that included the "modname:" prefix, and we should continue
to allow mods to do so, without registering the nodenames created
in the "doors:" namespace.

The default case is to use the "modname:" prefix verbatim. If
mods or code calls this function without a prefix, then "doors:"
is automatically used.

Now that the namespace is corrected, the copy replacement ABM is
no longer needed.