macipgw: Introduce a config file

[rdmark]

To implement the parsing of a macipgw.conf file where the various runtime options can be managed.
When CLI parameters are passed to macipgw, they should take precedence to the config file.

First order of business: Choose a config file parsing strategy. Off the top of my head I think we should use the iniparser library in libatalk.

Branched off from #1204