This feature was mainly for laptops but users:
- often don't care having the daemon running all the time;
- may not want to use the 'main-interface' option because the interface that
  provides internet access is not always the same, e.g. wifi & wired;
- may want other kinds of conditions,
  e.g. only specific wifi access points.

So in practice, main-interface is currently only use to set up a DHCP server or
provide IPv6 autoconfiguration. For such case, it is preferred to start/stop
re6stnet like a normal service.

The behaviour of --link-mtu has changed and we increase the values to
at least have interface MTU greater than IPv6 minimum.
We'll see later to have even greater values in ovpn_link_mtu_dict
(so that the resulting MTU is closer to what we had with 2.3)
or review the whole MTU part completely.

- Suse wants units in /usr/lib/systemd/system
- respect system-preset

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.

- new -V/--version command line option
- protocol extended to get the version of any node in the network,
  which will allow to track those running an old version of re6st