Support for custom fastd config files in /etc/fastd/

For our local freifunk we need an option to add configuration options to fastd. With these changes fastd will look in /etc/fastd/ (which is initially empty) for files and add them with 'include'. Other packages can add those files and insert options like 'on pre-up' to fastd.

To allow any possible option fastd is compiled with all FASTD_WITH_CMDLINE_* enabled.