reverse zone: fix Normalize (#4621)

Make normalize return multiple "hosts" (= reverse zones) when a
non-octet boundary cidr is given.

Added pkg/cidr package that holds the cidr calculation routines; felt
they didn't really fit dnsutil.

This change means the IPNet return parameter isn't needed, the hosts are
all correct. The tests that tests this is also removed: TestSplitHostPortReverse
The fallout was that zoneAddr _also_ doesn't need the IPNet member, that
in turn make it visible that zoneAddr in address.go duplicated a bunch
of stuff from register.go; removed/refactored that too.

Created a plugin.OriginsFromArgsOrServerBlock to help plugins do the
right things, by consuming ZONE arguments; this now expands reverse
zones correctly. This is mostly mechanical.

Remove the reverse test in plugin/kubernetes which is a copy-paste from
a core test (which has since been fixed).

Remove MustNormalize as it has no plugin users.

This change is not backwards compatible to plugins that have a ZONE
argument that they parse in the setup util.

All in-tree plugins have been updated.

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

plugin/loop/README.md

@@ -13,6 +13,8 @@ The plugin will try to send the query for up to 30 seconds. This is done to give
 to start up. Once a query has been successfully sent, *loop* disables itself to prevent a query of
 death.
 
+Note that *loop* will _only_ send "looping queries" for the first zone given in the Server Block.
+
 The query sent is `<random number>.<random number>.zone` with type set to HINFO.
 
 ## Syntax