Refactor marker-beacons to be distinct from FGNavRecord. This is a big space

Refactor marker-beacons to be distinct from FGNavRecord. This is a big space
saving for beacons, but since they're surprisingly few (in nav.dat), not a
an enormous saving in real terms. The major motivation is that marker beacons
don't behave like other NavRecords for radio interaction - they have no ident,
frequency or range (in the sense that NavRecord means them).