Default `ini` files provided by the nix package

[Atry]

Is your feature request related to a problem? Please describe.
Our native Debian/Ubuntu deb packages provide some default configuration files at /etc/hhvm/php.ini and /etc/hhvm/server.ini. However we did not provide the same files from the nix package.

Describe the solution you'd like
It would be good if we could provide the default configuration files for nix package as well.

Describe alternatives you've considered
I am not sure about the purpose of the default configuration files. The files are provided in native deb packages under /etc because they are system packages. A user can install the deb packages and then manually update the configuration files. However, nix packages do not follow Filesystem Hierarchy Standard, and the nix store is expected to be read only. Even if we provide the configuration files at /nix/store/xxxxxxxx-hhvm-xxxx/etc/hhvm, the users should not change it. For comparison, in our previously supported Homebrew bottles, the configurations files were under /usr/local/Cellar/hhvm/x.yyy/.bottle/etc/hhvm and were symbolically linked to /usr/local/etc.

Additional context
Given that we have universal deb packages packed from nix packages, should we adopt some part of Filesystem Hierarchy Standard in the universal deb package? Should we support /etc/hhvm/ path instead of /nix/store/xxxxxxxx-hhvm-xxxx/etc/hhvm path?