netsniff-ng: Display pcap I/O method in verbose mode

If a user accidentially specifies more than one of --mm/--sg/--clrw, the
option specified last will be used - as expected from standard command
line tools. In order to still prevent users from being confused by this,
explicitely display the pcap I/O method used in verbose mode.

In order for the output to be more user-friendly, actually write out the
method names in const char *pcap_ops_group_to_str, which isn't used
anywhere else anyway.

Suggested-by: Jon Schipp <jonschipp@gmail.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>