[aprssig] APRS Voice Alert

[Mark Saurman]

One of my trackers would transmit right overtop of a voice alert, and
I'd imagine so would a few others using a similar setup.  I have a
Kantronics TNC that is set to software carrier detect as the vehicle
travels into high interference areas which will hold the squelch open
for minutes at a time even though no signal is present.  If the
squelch is turned up, then it may miss weaker signals transmitting
simultaneously while outside of the interference area.  If a voice
station comes on, my TNC will not know it and just transmit over it if
the timer says it's time to transmit.