Do more fixups of the "phy" based on the data rate, so that it reflects
the modulation used for the packet.

Note, in comments, why we're doing this, and that there's no reiable
way, in radiotap, to determine the type of channel on which capturing is
being done, as some packet providers use the channel field to indicate
the channel type and others use it to indicate the modulation.

Only provide the "short preamble" for "11b", as that's now being used to
mean "DSSS modulation" - packets on an 11g channel will be marked as
"11g" if they're OFDM or "11b" if they're DSSS.

Make some other cleanups while we're at it.