Pull request: all: replace aghstrings with stringutil

Merge in DNS/adguard-home from add-stringutil to master

Squashed commit of the following:

commit 4ca9b29356de7d0a162b1e5a6496c691e9751f15
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Jul 29 17:29:25 2021 +0300

    all: replace aghstrings with stringutil

internal/dnsforward/http_test.go

@@ -241,6 +241,12 @@ func TestDNSForwardHTTP_handleSetConfig(t *testing.T) {
 	}
 }
 
+func TestIsCommentOrEmpty(t *testing.T) {
+	assert.True(t, IsCommentOrEmpty(""))
+	assert.True(t, IsCommentOrEmpty("# comment"))
+	assert.False(t, IsCommentOrEmpty("1.2.3.4"))
+}
+
 // TODO(a.garipov): Rewrite to check the actual error messages.
 func TestValidateUpstream(t *testing.T) {
 	testCases := []struct {