mausezahn: provide listen-addr option to configuration file

Specify the listen-addr for the CLI in /etc/netsniff-ng/mausezahn.conf
and remove the message, "mz: device not given, will use eth0" since
we're listening by IP, rather than interface now, and this shows continues
to show.

$ grep listen-addr /etc/netsniff-ng/mausezahn.conf
listen-addr = 127.0.0.1

$ mausezahn -V -x

 mz: device eth0 got assigned 10.0.2.15 and is a possible candidate.
 mz: device lo got assigned 127.0.0.1 (loopback)
Opening config file /etc/netsniff-ng/mausezahn.conf...
...
Mausezahn accepts incoming Telnet connections on 127.0.0.1:25542.

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