Pull request: all: imp dhcp client hostname normalization

Updates #2952.
Updates #2978.

Squashed commit of the following:

commit 20e379b94ccf8140fd9056429315945c17f711a5
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon Apr 19 15:58:37 2021 +0300

    all: imp naming

commit ed300e0563fa37e161406a97991b26a89e23903a
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon Apr 19 15:43:09 2021 +0300

    all: imp dhcp client hostname normalization

internal/aghnet/addr_test.go

@@ -88,6 +88,14 @@ func TestValidateDomainName(t *testing.T) {
 		name:       "success_idna",
 		in:         "пример.рф",
 		wantErrMsg: "",
+	}, {
+		name:       "success_one",
+		in:         "e",
+		wantErrMsg: "",
+	}, {
+		name:       "empty",
+		in:         "",
+		wantErrMsg: `domain name is empty`,
 	}, {
 		name: "bad_symbol",
 		in:   "!!!",
@@ -111,6 +119,11 @@ func TestValidateDomainName(t *testing.T) {
 		in:   "example.-aa.com",
 		wantErrMsg: `invalid domain name label at index 1:` +
 			` invalid char '-' at index 0 in "-aa"`,
+	}, {
+		name: "bad_label_last_symbol",
+		in:   "example-.aa.com",
+		wantErrMsg: `invalid domain name label at index 0:` +
+			` invalid char '-' at index 7 in "example-"`,
 	}, {
 		name: "bad_label_symbol",
 		in:   "example.a!!!.com",
@@ -166,7 +179,7 @@ func TestGenerateHostName(t *testing.T) {
 
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
-			hostname := GenerateHostName(tc.ip)
+			hostname := GenerateHostname(tc.ip)
 			assert.Equal(t, tc.want, hostname)
 		})
 	}