Pull the record processing into netmon_process_record().

That combines more common code from the read and seek-read code
paths.

Also, separate out the individual metadata record types, with a comment
for each, to simplify the process of supporting some or all of them in
the future.

Change-Id: Ic8ded397d9550ec6013c1f5f138333b1ef5c37e5
Reviewed-on: https://code.wireshark.org/review/2869
Reviewed-by: Guy Harris <guy@alum.mit.edu>