dns/nsd: Remove IPv6 option, fixes build

Remove the IPv6 option that is causing builds to fail when it is
disabled. The issue does not affect package users, as it was a default
option.

The issue has been fixed upstream [1] and will be included/renabled
in the next version update.

While I'm here:

• Switch to USES=ssl
• Add --enable-ipv6 in CONNFIGURE_ARGS to ensure it's explicitly enabled

[1] https://www.nlnetlabs.nl/bugs-script/show_bug.cgi?id=800

PR: 211303
Reported by: <vfx9as gmail com>
Approved by: maintainer <jaap NLnetLabs nl>
MFH: 2016Q3