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/k8s_external/setup.go

@@ -44,15 +44,7 @@ func parse(c *caddy.Controller) (*External, error) {
 	e := New()
 
 	for c.Next() { // external
-		zones := c.RemainingArgs()
-		e.Zones = zones
-		if len(zones) == 0 {
-			e.Zones = make([]string, len(c.ServerBlockKeys))
-			copy(e.Zones, c.ServerBlockKeys)
-		}
-		for i, str := range e.Zones {
-			e.Zones[i] = plugin.Host(str).Normalize()
-		}
+		e.Zones = plugin.OriginsFromArgsOrServerBlock(c.RemainingArgs(), c.ServerBlockKeys)
 		for c.NextBlock() {
 			switch c.Val() {
 			case "ttl":