altos: Re-send previous GPS position in APRS if lock is lost

APRS radios often show only the last received APRS packet, which means
that erasing the last known GPS position when we lose lock by sending
0/0/0 is unhelpful. Instead, just send the last known position, and
make sure that we send 0/0/0 before we're locked the first time.

Signed-off-by: Keith Packard <keithp@keithp.com>