* middleware/httproxy: implement debug queries

Not too useful at the moment, but o-o.debug queries are supported
and return the Comment from dns.google.com.

Note that this is not always set.

* improve documentation

* Testing cleanups

Also release ARM binary when releasing. Comes in handy for raspberry Pi.

This PR adds a middleware that talks to dns.google.com over HTTPS,
meaning all your DNS traffic is encrypted when traversing your ISP and
the internet.

The `dns.google.com` address is re-resolved every 30 seconds.

* middleware/proxy: config syntax cleanups

Allow port numbers to be used in the transfer statements and clean
up the proxy stanza parsing. Also allow, when specifying an upstream,
/etc/resolv.conf (or any other file) to be used for getting the upstream
nameserver.

Add tests and fix the documentation to make clear what is allowed.

* Fix the other upstream parse as well

middleware/file: better notify error reporting

Report errors as originating from notifies in all cases.

Fixes #412

Propagate both message and error in proxy exchange.

K8s Test Cleanup and Service PTR

middleware/auto: Close to reader

Close the reader, stop leaking file descriptors.

Change the CI setup for K8s to be simpler. Now it just creates a
set of objects via a yaml file, making it very easy to modify
the tests.

Implement PTR for services.

Provide TTL examples for cache middleware. Fixes: #364

Add interaction docs to make non invalidation for cache middleware
clear. Cache might serve stale records. Fixes: #403

Provide TTL examples for cache middleware. Fixes: #364

Add interaction docs to make non invalidation for cache middleware
clear. Cache might serve stale records. Fixes: #403

Fixes several typos in docs.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

This fix adds Codecov badge to README.md so that coverage data shows up.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Fix typos

* Simplify code

* Fix error usage

Only manual operation is editing the source - even that could be
automated but meh.

* When no records match, reply with NXDOMAIN

* Implement in IsNameError

* case for unexposed namespace. k8s integation tests

* Fix imports order. Lower case of err strs.

This reverts commit 15ab38f0.

Add type to default template

Do the same thing as in etcd and give the option of externally resolving
CNAME. This is needed when CoreDNS is a proxy as well is serving zones.

* middleware/file: also react to rename events.

* React to all events

Up till now we would only chase 1 CNAME. Spec requires we will chase
all. This PR add support for this.  Up to 8 CNAMEs are chased (this
could be longer, by just checking for cycles, but 8 seems enough for
now).

Also add RRSIG of the first CNAME for DNSSEC.

* middleware/cache: cache 0 will be capped at 5

cache 0 would return TTL=0 records, up that to the documented minimum of
5 seconds.

* middleware/cache: check for 0 TTL

Handle 0 TTL differently and return an error, we might need to
special case this in the future.