IPv6 support for master-minion communication

These commits add proper IPv6 support to the minion-master communication path. The system hostname lookup mechanisms are used to determine whether v6 or v4 is preferred, which should prevent any detriment to IPv4-only setups.

I've tested these patches in an IPv6-only setup, an IPv4-only setup and a mixed setup, and the hosts seem to communicate fine in all cases.

When specifying an IPv6 listen address, it has to be quoted, since : interferes with YAML syntax. I'm using the following in my master config:

    interface: '::'