[aprssig] APRS callsign minimum length

[Bob Bruninga]

>> (Remember that APRS callsigns cannot be 
>> shorter than 3 bytes).
>
> Where in the spec is this mentioned?

I donno.  If you can't find it in the spec, it should be in the aprs11 addendum www.aprs.org/aprs11.html

If you cant find it there lemme know and we'll add it.