[aprssig] Excess Tx I-gating

[John Langner WB2OSZ]

The direwolf IGate allows very flexible filtering to satisfy a wide range of
special situations.

Filter specifications can select packets based on source address,
destination address, digipeater addresses (used or unused), APRS packet type
(weather, telemetry, etc.), symbols, and distance from a location.   They
are loosely modeled after the Server Side filters.   These can be combined
into complex expressions including AND, OR, NOT operators, and grouping
parentheses for changing evaluation order.

In the absence of an IS>RF filter, the default is APRS "Messages" with
addressee matching a station heard recently on RF.  The next position
report, from the message sender, is also transmitted.

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

The original posting asked about "forwarding geographically local
Internet-only stations to RF."  Based on the documentation
(http://www.aprs-is.net/q.aspx) and observations, we probably want to look
for "qAC" in the digipeater list.  For example, this was observed coming
from an APRS-IS server:

N1LMA>APU25N,TCPIP*,qAC,T2NUENGLD:@250058z4123.63N/07148.85W_094/001g159t042
r002p003P003h98b10253CRSnet {UIV32N}

The complete filter might look something like this:

i/30 | ( v/qAC & r/42.6/-71.3/50 )

We use the v filter, rather than the d filter, because the "qAC" address is
not marked as being used (i.e. "*" does not appear after it).