Extend the ax25 timers, maxpacket, etc fields to be per interface

[erikarn]

right now a bunch of ax25 configuration parameters are global rather than per interface. This has the unfun side effect of making it less interesting to run a single ka9q instance with multiple TNCs spanning 1200 VHF, 9600 baud UHF and 300 baud HF.

The easiest way to address this would be to move a bunch of configuration to be per interface and configured using ifconfig. it means ifconfig will need to grow some per-linklevel state that gets copied over into the ax25 control block upon creating a connection but i think it's doable.