Explicit reply w/o ACK to confirmed uplink is not handled

[terrillmoore]

The latest EU compliance tests check the following.

1. Device issues a confirmed uplink
2. network replies with a downlink in RX1 or RX2 but FHDR.FCtrl.ACK bit is reset
3. device is supposed to notice that ACK is not set, and retransmit.

In fact, LMIC just assumes that a valid downlink was an ACK. That's probably a reasonable assumption, but it's not compliant with the spec.

Apart from this, the LMIC passes the entire EU V1.5 test, except for the optional data rates.

[terrillmoore]

While fixing this, we should also do #238, because otherwise this will involve adding yet another goto to processDnData(). Since the goto's end up expressing pure tail-recursion rather than iteration, any compiler is going to generate pretty much the same code anyway.