The read event demultiplexer

    select(2) on Unix FDs
        ...
    Fill up AX.25 interface packet buffers
        ...
    Decode as many packets as possible on each interface
        ...
    Fill up buffers for pending caller reads, open new connections
        ...
    If:
        * ax25_select() is called: Notify caller of I/O events

        * ax25_read() is called: Provide caller access to requested
          buffer region

    Buffer may be discarded upon next ax25_read() event, but should not
    be discarded after an ax25_select() necessarily.