Capability: a receive-only IGate #206

Goal: aprscli serve can upload what its TNCs hear to APRS-IS as a receive-only IGate, with the gating rules and q-constructs APRS-IS expects. The gateway already reads TNCs and holds an APRS-IS login, so most of the work is the rules.

This starts AprsNet.IGate with the RF-to-internet direction. 0.7.0 adds the internet-to-RF direction that makes it a full IGate.

Receive-only IGates are known to break messaging: stations nearby think they can be reached through the IGate, but messages never come back to them on RF. The docs say so, and point to the two-way IGate in 0.7.0.

Done when:

• a live run with a verified login uploads packets heard on RF, and they show on aprs.fi with qAO (aprs-is.net: receive-only IGates use qAO, not qAR) and the IGate's call;
• the IGate beacons its own position to APRS-IS, as IGates are expected to;
• it is in a release.