* DoH: put in pkg/doh

Factor out the DoH stuff into its own package, add function to request
a DoH response. This can be used by forward (and maybe proxy) to
implement DoH client support.
Signed-off-by: Miek Gieben <miek@miek.nl>

* lint
Signed-off-by: Miek Gieben <miek@miek.nl>

* ... and make it compile
Signed-off-by: Miek Gieben <miek@miek.nl>