tshark: Print the packets' comments in the expert info

tshark: Print the packets' comments in the expert info

Previously 'tshark -z expert' was failing with abort when a packet
contains a comment

- Add a new comment parameter and update the tshark's manual page
- Add a new comment_level severity and change the default lavel to it.
- Add various 'tshark -z expert' tests

Change-Id: I188317da5e00019b8f2b725f0fe84942f774520f
Reviewed-on: https://code.wireshark.org/review/30610
Petri-Dish: Guy Harris <guy@alum.mit.edu>
Tested-by: Petri Dish Buildbot
Reviewed-by: Guy Harris <guy@alum.mit.edu>