ctclient: allow DNS nameserver specification
ctclient: allow DNS nameserver specification

RJPercival/certificate-transparency-go,daviddrysdale/certificate-transparency-go,RJPercival/certificate-transparency-go,RJPercival/certificate-transparency-go,daviddrysdale/certificate-transparency-go,google/certificate-transparency-go,daviddrysdale/certificate-transparency-go,google/certificate-transparency-go,google/certificate-transparency-go