Add tshark -G dissector-tables to dump a list of dissector tables.

That list doesn't show the entries in the dissector tables, just
information about the tables themselves.

Clean up some tshark man page issues while we're at it.

Change-Id: I70beee34110f5c0d58105944dd71105a8400f5ca
Reviewed-on: https://code.wireshark.org/review/5360
Reviewed-by: Guy Harris <guy@alum.mit.edu>