start isc-dhcp-server for IPv6

Andrew Bowerrequested to merge
abower/runit-services:isc-dhcp-server-ipv6 into next
  1. Adds a runscript to enable the ISC dhcp server for IPv6. This restores parity with the initscript.
  2. Ensures the leases file is created before starting the server. This is a required step but my not have been noticed for IPv4 due to the initscript creating this, probably before the runscript has taken over from it.
  3. Adds an empty environment file for the INTERFACESvX config to make it more obvious to the user how to set this, what with the /etc/default-based environment now being ignored.
  4. Aborts and disables service if no interfaces have been specified, to stop this happening with a soft failure later, repeatedly. (The config check doesn't do this.)

I think this dual runscript is a good case for it being better to colocate with the main daemon package - it ought to be neater - the two scripts are not consistently named (e.g. .4, .6 but one of them has to match the package name for the service management logic to work here.

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