UltraProbe's tryno and pingseq attributes were mutually exclusive,
encoded in the same places in packets, and did not need more than 7 bits
to store. This change combines them into a bitfield. This simplifies
some logic, since they do not need to be distinguished from each other.
Additionally, in cases where the tryno is encoded in the source port
number, no further checks or decoding need to happen beyond comparing
the destination port of the response to the source port of the probe.