Pull request: all: fix lint and naming issues vol. 4

Merge in DNS/adguard-home from 2276-fix-lint-4 to master

Updates #2276.

Squashed commit of the following:

commit 15d49184cd8ce1f8701bf3221e69418ca1778b36
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Dec 8 15:51:34 2020 +0300

    util: fix naming

commit 3b9a86a0feb8c6e0b167e6e23105e8137b0dda76
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Dec 8 15:41:10 2020 +0300

    all: fix lint and naming issues vol. 4

internal/agherr/agherr.go

@@ -65,3 +65,9 @@ func (e *manyError) Unwrap() error {
 
 	return e.underlying[0]
 }
+
+// wrapper is a copy of the hidden errors.wrapper interface for tests, linting,
+// etc.
+type wrapper interface {
+	Unwrap() error
+}

internal/agherr/agherr_test.go

@@ -41,6 +41,8 @@ func TestError_Error(t *testing.T) {
 }
 
 func TestError_Unwrap(t *testing.T) {
+	var _ wrapper = &manyError{}
+
 	const (
 		errSimple = iota
 		errWrapped