Default `ini` files provided by the nix package

**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?

Inspect the nix package definition and universal deb packaging, using the Debian/Ubuntu paths `/etc/hhvm/php.ini` and `/etc/hhvm/server.ini` as the reference. Determine whether read-only nix-store files or an `/etc/hhvm/` integration is intended; done means the packaging behavior and configuration paths are explicitly decided and implemented.

Written by the indexing model from the issue text.