namesys/dns: Use SplitN to find dnslink references

RFC 6763 requires printable ASCII except '=' for the key [1], but
allows any character including '=' in the value [2].  This patch
adjusts our parsing to avoid splitting on '=' in the value, and then
ignoring anything after that split.

[1]: https://tools.ietf.org/html/rfc6763#section-6.4
[2]: https://tools.ietf.org/html/rfc6763#section-6.5