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/metrics/setup.go

@@ -80,8 +80,9 @@ func parse(c *caddy.Controller) (*Metrics, error) {
 		}
 		i++
 
-		for _, z := range c.ServerBlockKeys {
-			met.AddZone(plugin.Host(z).Normalize())
+		zones := plugin.OriginsFromArgsOrServerBlock(nil /* args */, c.ServerBlockKeys)
+		for _, z := range zones {
+			met.AddZone(z)
 		}
 		args := c.RemainingArgs()