Split patty_ax25_if_recv() into the following parts:
* patty_ax25_if_fill(), to fill the internal PHY buffer
* patty_ax25_if_drain(), to drain the internal PHY buffer and decode
any data present in the buffer
* patty_ax25_if_ready(), to test if a complete frame has been
decoded
* patty_ax25_if_flush(), to return the final number of bytes decoded
in the PHY buffer and reset the decoding state
Other changes:
* Implement a finite state machine for decoding KISS traffic in
src/tnc.c, patty_ax25_kiss_drain()
* Refactor src/server.c, handle_iface() to use the aforementioned
frame receive/decode methods
d5e2062dea