Add backend support for HE.net AAAA record updates.

Defines a new DynDNS provider 'he-net-v6' for updating AAAA entries on
dns.he.net.
Clean up HE.net AAAA backend support.
Add front-end support for dyndns AAAA updates
Merge branch 'master' into dyndns_custom_v6
Change separator as per JimP's request.
Add support for custom IPv6 DDNS.
Merge pull request #703 from razzfazz/dyndns_custom_v6

Add support for HE.net AAAA record updates. Fixes #1825.