Answer REFUSED when we don't host the zone
Answer REFUSED when we don't host the zone

Change from SERVFAIL to diff. between error and no such zone hosted.

Fixes: #68

yongtang/coredns,coredns/coredns,Infoblox-CTO/coredns,johnbelamaric/coredns,jremond/coredns,Infoblox-CTO/coredns,jremond/coredns,ii/coredns,ii/coredns,Infoblox-CTO/coredns,chrisohaver/coredns,miekg/coredns,varyoo/coredns,johnbelamaric/coredns,miekg/coredns