go/client/ctclient: add getroots command
go/client/ctclient: add getroots command

Also start shifting towards having a ctx as first argument
everywhere (populated in main()), in preparation for #1326.

phad/certificate-transparency,lexibrent/certificate-transparency,grandamp/certificate-transparency,phad/certificate-transparency,lexibrent/certificate-transparency,pphaneuf/certificate-transparency,lexibrent/certificate-transparency,phad/certificate-transparency,grandamp/certificate-transparency,RJPercival/certificate-transparency,grandamp/certificate-transparency,RJPercival/certificate-transparency,google/certificate-transparency,grandamp/certificate-transparency,pphaneuf/certificate-transparency,google/certificate-transparency,RJPercival/certificate-transparency,pphaneuf/certificate-transparency,RJPercival/certificate-transparency,google/certificate-transparency,pphaneuf/certificate-transparency,lexibrent/certificate-transparency,google/certificate-transparency,phad/certificate-transparency