IPv6 support

[naggie]

• [x] choose ULA network
• [x] check is allocated
• [x] allocate v6
• [x] move allocation out of loop (IPv4 allocation) (also wtf is going on there anyway?)
• [x] template generation
• [x] only allocate if network is defined for v4 and v6
• [x] address subnets should be in generated config
• [x] check for public ipv6
• [x] check init with and without IPv6
• [x] check there's an external IPv4 or IPv6 in config after looking up IPs
• [x] endpoint: use IPv6 if IPv4 is not available
• [x] fix saving as encoded
• [x] fix cidrmask for address in config (!?!?)
• [x] fix nil pointer dereference when parsing empty Network JSONIPnet (add)
• [x] only write allowed IPs that exist for peers (IPv4, ipv6) (Netoerk, Netowrk6)
• [x] with statements in templates invalid, correct
• [x] check vyatta/wg-quick configs are coherent
• [x] fix Network6": "\u003cnil\u003e", -- happens when omitted and then saved
• [x] printpeerctg and getwgpeerconfigs should add v4 and v6 conditionally
• [x] update getwgpeerconfigs !
• [x] update docs with v6 instrucitons (NAT, no NAT, etc)

[naggie]

Actually I'll switch to random allocation for IPv6, as per the suggestion in the RFC. Almost forgot....

[naggie]

Thanks @frillip