Support the bandwidth and "short GI" fields from Peek tagged files.

Add a number of fields corresponding to components of the radiotap MCS
field, add presence bits for them, and set and dissect the fields
supported by Peek tagged files.

Change-Id: I3fc801a3bc180e1c174d074a794af0f3d338f249
Reviewed-on: https://code.wireshark.org/review/4989
Reviewed-by: Guy Harris <guy@alum.mit.edu>