[nEdge] Add IPv6 support

[lucaderi]

nEdge is currently IPv4 centric and IPv6 traffic is not currently handled. It would be nice to add support for it as people are starting to reply it in various networks,

[emanuele-f]

Here is a (possibly partial) checklist:

• use ip6tables to insert rules
• keep both ipv4 and ipv6 local ips
• add ipv6 support in captive portal
• conntrack: read ipv6 fields
• GUI: add the ability to specify a ipv6 addresses and configuration (possible both ipv4 and ipv6)
• Configure and start DHCPv6 server
• ipv4_utils: support for operations on ipv6 addresses
• nDPI: ndpi_add_string_value_to_automa (and relevant category matching structures/functions) must be extended to handle ipv6

[bloodyle]

Voting for this.
We are getting more and more IPv6 traffic and I believe it's crucial for NTOP/NEDGE to support analytics.