Pull request: all: mv some utilities to netutil

Merge in DNS/adguard-home from mv-netutil to master

Squashed commit of the following:

commit 5698fceed656dca7f8644e7dbd7e1a7fc57a68ce
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon Aug 9 15:44:17 2021 +0300

    dnsforward: add todos

commit 122fb6e3de658b296931e0f608cf24ef85547666
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon Aug 9 14:27:46 2021 +0300

    all: mv some utilities to netutil

internal/aghnet/hostgen_test.go

@@ -0,0 +1,48 @@
+package aghnet
+
+import (
+	"net"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestGenerateHostName(t *testing.T) {
+	testCases := []struct {
+		name string
+		want string
+		ip   net.IP
+	}{{
+		name: "good_ipv4",
+		want: "127-0-0-1",
+		ip:   net.IP{127, 0, 0, 1},
+	}, {
+		name: "bad_ipv4",
+		want: "",
+		ip:   net.IP{127, 0, 0, 1, 0},
+	}, {
+		name: "good_ipv6",
+		want: "fe00-0000-0000-0000-0000-0000-0000-0001",
+		ip:   net.ParseIP("fe00::1"),
+	}, {
+		name: "bad_ipv6",
+		want: "",
+		ip: net.IP{
+			0xff, 0xff, 0xff, 0xff,
+			0xff, 0xff, 0xff, 0xff,
+			0xff, 0xff, 0xff, 0xff,
+			0xff, 0xff, 0xff,
+		},
+	}, {
+		name: "nil",
+		want: "",
+		ip:   nil,
+	}}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			hostname := GenerateHostname(tc.ip)
+			assert.Equal(t, tc.want, hostname)
+		})
+	}
+}