Reintroduce an old bugture: use localizer as dme

Reintroduce an old bugture: use localizer as dme

Our nav.dat.gz seem to have many bugs regarding paired LOC/DME where
the DME is missing.

A feature has recently been introduced to always use a LOC frequency
as a DME to avoid these issues. However many LOC station do not have
a DME paired at their frequency and this behaviour might be as confusing
as having no DME where one should be.
Before we get new/bugfree navdata, a property can be set to select
the desired behaviour:

if /sim/realism/dme-fallback-to-loc is true or absent, LOC will be
treated as DME (realism is off)
if /sim/realism/dme-fallback-to-loc is false, LOC will be NOT be
treated as DME (realism is on)