Address Jon's comments

[miekg]

Address more (all?) of Jon's comments.

[mrichmon]

Changes look good.

Not sure about the phrasing of a "hard dependency" on Caddy. The new coredns.go partly addresses this. I view Caddy as a framework that coredns uses in two ways:

• much of the coredns code plugs into the framework to add dns behavior.
• coredns provides a wrapper around the framework to provide a dns-tuned command-line interface.
  So this feels like using a library/framework.