nexedi / re6stnet

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.