plugin/host: don't append the names when reparsing hosts file (#3045)

The host plugin kept on adding entries instead of overwriting. Split the
inline cache off from the /etc/hosts file cache and clear /etc/hosts
file cache and re-parsing.

A bunch of other cleanup as well. Use functions defined in the plugin
package, don't re-parse strings if you don't have to and use To4() to
check the family for IP addresses. Fix all test cases a carried entries
are always fqdn-ed. Various smaller cleanup in unnessacry constants.

Fixes: #3014

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

plugin/hosts/hosts_test.go

@@ -2,7 +2,6 @@ package hosts
 
 import (
 	"context"
-	"io"
 	"strings"
 	"testing"
 
@@ -12,20 +11,17 @@ import (
 	"github.com/miekg/dns"
 )
 
-func (h *Hostsfile) parseReader(r io.Reader) {
-	h.hmap = h.parse(r)
-}
-
 func TestLookupA(t *testing.T) {
 	h := Hosts{
 		Next: test.ErrorHandler(),
 		Hostsfile: &Hostsfile{
 			Origins: []string{"."},
-			hmap:    newHostsMap(),
+			hmap:    newMap(),
+			inline:  newMap(),
 			options: newOptions(),
 		},
 	}
-	h.parseReader(strings.NewReader(hostsExample))
+	h.hmap = h.parse(strings.NewReader(hostsExample))
 
 	ctx := context.TODO()