Expand the 802.11 pseudo-header and support new radio metadata.

Add a set of presence bits, so we can indicate which bits of radio
metadata we do and don't have.

Fill in more radio metadata from capture files, and display it.

(More to come.)

Change-Id: Idea2c05442c74af17c14c4d5a8d8025ab27fbd15
Reviewed-on: https://code.wireshark.org/review/4987
Reviewed-by: Guy Harris <guy@alum.mit.edu>