Pull request: all: imp dhcp host normalization, validation

Updates #2952.

Squashed commit of the following:

commit 45afcab5d33c1ec8176c0ad05423288c8770b772
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Apr 20 15:02:34 2021 +0300

    all: imp docs

commit d844ce1e2bb0b92a892119161774ec95f3e59711
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Apr 20 14:57:49 2021 +0300

    all: more code imp

commit eef08cb69015ead0ffaef866e12a2c6556786d37
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Apr 20 14:52:33 2021 +0300

    all: imp code, docs

commit 20748f20ab9fcc410b11daf0e23eaf156dcdc7cd
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Apr 20 14:30:57 2021 +0300

    all: imp dhcp host normalization, validation

internal/aghstrings/set_test.go

@@ -0,0 +1,56 @@
+package aghstrings
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestSet(t *testing.T) {
+	const s = "a"
+
+	t.Run("nil", func(t *testing.T) {
+		var set *Set
+
+		assert.NotPanics(t, func() {
+			set.Del(s)
+		})
+
+		assert.NotPanics(t, func() {
+			assert.False(t, set.Has(s))
+		})
+
+		assert.NotPanics(t, func() {
+			assert.Equal(t, 0, set.Len())
+		})
+
+		assert.NotPanics(t, func() {
+			assert.Nil(t, set.Values())
+		})
+
+		assert.Panics(t, func() {
+			set.Add(s)
+		})
+	})
+
+	t.Run("non_nil", func(t *testing.T) {
+		set := NewSet()
+		assert.Equal(t, 0, set.Len())
+
+		ok := set.Has(s)
+		assert.False(t, ok)
+
+		set.Add(s)
+		ok = set.Has(s)
+		assert.True(t, ok)
+
+		assert.Equal(t, []string{s}, set.Values())
+
+		set.Del(s)
+		ok = set.Has(s)
+		assert.False(t, ok)
+
+		set = NewSet(s)
+		assert.Equal(t, 1, set.Len())
+	})
+}