Pull request: dhcpd: assume static ip on eperm

Merge in DNS/adguard-home from 2667-eperm-dhcp to master

Updates #2667.

Squashed commit of the following:

commit 7fad607ae0ae75419005707ee58312bc64fe78c5
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Fri Feb 12 16:27:59 2021 +0300

    dhcpd: assume static ip on eperm

internal/dhcpd/http_test.go

@@ -0,0 +1,23 @@
+package dhcpd
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestServer_notImplemented(t *testing.T) {
+	s := &Server{}
+	h := s.notImplemented("never!")
+
+	w := httptest.NewRecorder()
+	r, err := http.NewRequest(http.MethodGet, "/unsupported", nil)
+	require.Nil(t, err)
+
+	h(w, r)
+	assert.Equal(t, http.StatusNotImplemented, w.Code)
+	assert.Equal(t, `{"message":"never!"}`+"\n", w.Body.String())
+}