debian/rules is split into reusable parts for SlapOS-based packages.

The old distutils way is not compatible with zc.recipe.egg in develop mode,
because egg_info does not provide any information about such scripts.

2.2.x branch has a patch[1] that renders tls_serial_* environment variables
in base 16, causing a ValueError exception in ovpn-server hook.

[1] https://github.com/OpenVPN/openvpn/commit/7d5e26cbb53e2700c966e6b6e815f0c824da8956

This simplify network configuration a lot, and on recent kernels, this fixes
wrong source address for extra interfaces that already have a public IP.