Allow specifying the path of /etc/os-release

/etc/os-release is a specification invented on Linux. It is adopted by most Linux distributions, but it haven't been considered important enough for other non-Linux systems to implement it. On FreeBSD, the base system doesn't include an os-release file. To get an os-release file, users can install an additional package called 'etc_os-release'. Since it isn't considered as a part of the system, it can't be put in /etc and gnome-control-center is unable to find it.

Therefore, to allow gnome-control-center finding os-release file in a different location, we have to add an option to allow changing the default path. An alternative solution is to use 'uname' function when os-release can't be opened, but the proposal was rejected.