[aprssig] IGate design theory

[John Langner WB2OSZ]

Inspired by the IGate discussions during the past 2 months, the Dire Wolf
IGate implementation has been revisited and improved.

The new default behavior, with no special IS-to-RF filter supplied, is to
transmit only "messages" addressed to stations recently heard nearby and the
next position report from the sender of the message.

Very flexible client side filtering allows customization for special
situations.

More rambling here:
https://github.com/wb2osz/direwolf/raw/dev/doc/Successful-APRS-IGate-Operati
on.pdf

Thanks to KB3TBX for raising some interesting questions and making many
suggestions for improvements.