Synchronet doesn't know that FTN Zones 1-4 are all fidonet

When choosing a source FTN address from the list of system addresses, Synchronet looks for exact matches between nets and zones and then just exact matches for zones.

The request is that for an example system that has an fsxNet (zone 21) address as their default address and a FidoNet zone 1 address as a secondary address, when replying to a netmail from zone 2, automatically use the zone 1 address, not zone 21. Since zone 1 mathematically "closer" to zone 2 than zone 21 is, this seems like an easy algorithm to implement (in nearest_sysfaddr()).