[aprssig] APRS device identifiers (REPLY-ACK)

[Robert Bruninga]

> APRS+SA is still the best/handiest client application for
keyboard-keyboard APRS messaging...

That's because it, like APRSdos, implemented the REPLY-ACK technique which
was never implemented in many other clients at the time.  The REPLY-ACK
embeds ACKS in reply messages so that the reliability of message delivery
and ACK response is vastly improved in two-way QSO's.

Bob, WB4APR