API: return status 204 on successful delete

204 No Content is meant to indicate that the data (zone) is now gone.

Also add tests to make sure DELETE works.

Fixes #1422.
stay away from getaddrinfo unless we really can't help it for ascii ipv6 conversions to binary
Merge pull request #1427 from zeha/api-delete-204

API: return status 204 on successful delete
fix an important omission from the docs
Support for PKCS#11 signing