mausezahn: re-work configuration file

/etc/netsniff-ng/mausezahn.conf as mentioned in manual was never read
because code was statically set to mz.cfg in /etc/mausezahn. This patch
simplifies things by using the existing /etc/netsniff-ng directory created
in the default install and still using a single configuration but with a
an easier to remember name e.g. ${tool}.conf.

Example:

$ cat /etc/netsniff-ng/mausezahn.conf
user = testuser
password = testpassword
enable = awesome

Pre-patch:

$ mausezahn -x
Mausezahn accepts incoming Telnet connections on port 25542.
mz: Problems opening config file. Will use defaults

Post-patch:

$ mausezahn -x
Mausezahn accepts incoming Telnet connections on port 25542.

Signed-off-by: Jon Schipp <jonschipp@gmail.com>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>