Implement AX.25 v2.2, Section 6.4.6 "Receiving Acknowledgement"
semantics more consistently
Changes:
* Add frame_ack() method in src/server.c to unify logic for
acknowledging frames and handling I frame transmission flow
control
* Ensure that the Timer T1 retry counter is reset as appropriate
when Timer T1 is restarted
* Add excerpts from the AX.25 v2.2 documentation to accompanying
code to better document the purpose of important aspects of the
state machine and protocol
* When receiving an S frame with N(R) acknowledgement of a frame
sent earlier than indicated by V(S), set V(S) to that N(R) value
* Implement patty_ax25_sock_resend_pending() as a means to resend
one frame previously sent which remains unacknowledged; this
uses V(S) to determine the frame to resend, and increments V(S) if
there was indeed an unacknowledged frame
* Call patty_ax25_sock_resend_pending() in src/server.c, method
handle_sock() to send a single frame previously sent but pending
acknowledgement
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