Add pkg/fall for Fallthrough (#1355)

* Add pkg/fall for Fallthrough

Move this into it's own package to facilitate tests. Important bug
was fixed: make the names fully qualified.

Add fall package to hosts, reverse, etcd, and fix kubernetes and any
tests. The k8s tests are still as-is, might need a future cleanup.

plugin/pkg/fall/fall.go

@@ -0,0 +1,77 @@
+// Package fall handles the fallthrough logic used in plugins that support it.
+package fall
+
+import (
+	"github.com/coredns/coredns/plugin"
+)
+
+// F can be nil to allow for no fallthrough, empty allow all zones to fallthrough or
+// contain a zone list that is checked.
+type F []string
+
+// New returns a new F.
+func New() *F { return new(F) }
+
+// Through will check if we should fallthrough for qname. Note that we've named the
+// variable in each plugin "Fall", so this then reads Fall.Through().
+func (f *F) Through(qname string) bool {
+	if f == nil {
+		return false
+	}
+	if len(*f) == 0 {
+		return true
+	}
+	zone := plugin.Zones(*f).Matches(qname)
+	return zone != ""
+}
+
+// SetZones will set zones in f.
+func (f *F) SetZones(zones []string) {
+	for i := range zones {
+		zones[i] = plugin.Host(zones[i]).Normalize()
+	}
+	*f = zones
+}
+
+// Example returns an F with example.org. as the zone name.
+var Example = func() *F {
+	f := F([]string{"example.org."})
+	return &f
+}()
+
+// Zero returns a zero valued F.
+var Zero = func() *F {
+	f := F([]string{})
+	return &f
+}
+
+// IsNil returns true is f is nil.
+func (f *F) IsNil() bool { return f == nil }
+
+// IsZero returns true is f is zero (and not nil).
+func (f *F) IsZero() bool {
+	if f == nil {
+		return false
+	}
+	return len(*f) == 0
+}
+
+// Equal returns true if f and g are equal. Only useful in tests, The (possible) zones
+// are *not* checked.
+func (f *F) Equal(g *F) bool {
+	if f.IsNil() {
+		if g.IsNil() {
+			return true
+		}
+		return false
+	}
+	if f.IsZero() {
+		if g.IsZero() {
+			return true
+		}
+	}
+	if len(*f) != len(*g) {
+		return false
+	}
+	return true
+}