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
		
	
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