Implement patty_ax25_aprs_is_send() to be able to encode any outbound UI
frames in TNC2 format, and to send them to a remote APRS-IS server
Other changes:
* Remove unimplemented patty_ax25_aprs_is_printf() declaration
* Remove unimplemented patty_ax25_aprs_is_readline() declaration
Implement an APRS-IS interface type which is capable of receiving all
APRS-IS traffic which can be represented in AX.25 frames; currently,
there are no provisions for handling extended features such as
non-numeric SSIDs, nor a client nor server which facilitate direct
APRS-IS communication. Presently, APRS-IS traffic cannot be sent from
this interface.
Changes:
* Add patty_ax25_aprs_is driver type in src/aprs_is.c
* Implement 'aprs-is' interface configuration type in bin/if.c; make
separate, smaller state machines for handling base interface
config, KISS, and APRS-IS interface config
