RF-to-internet gating rules #207

Decide whether a packet heard on RF goes to APRS-IS, and how.

Scope:

• Don't gate: paths with TCPIP, TCPXX, NOGATE or RFONLY; third-party packets already gated; generic queries; packets that fail to parse.
• Add qAR,<igate call> and strip what the IGate must not pass on.
• Duplicate suppression with DuplicateFilter, so a packet heard through several digipeaters goes up once.
• A reason for every packet not gated, for logs and metrics.

Done when:

• table tests cover each rule, from corpus lines and the aprs-is.net IGate documentation;
• metrics count gated and refused packets by reason;
• the API is listed in PublicAPI.Unshipped.txt.