Pull request: all: rm var shadowing, vol. 1

Updates #2803.

Squashed commit of the following:

commit 15fa64ad4fb7561c4b0542245f99869685643bbd
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Mar 11 14:55:28 2021 +0300

    util: imp code

commit c3b0563a44ccc98a5901df19174acdca8a350d62
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Mar 11 14:38:54 2021 +0300

    all: rm var shadowing, vol. 1

internal/util/helpers.go

@@ -94,7 +94,8 @@ func IsOpenWRT() bool {
 			continue
 		}
 
-		body, err := ioutil.ReadFile(filepath.Join(etcDir, fileInfo.Name()))
+		var body []byte
+		body, err = ioutil.ReadFile(filepath.Join(etcDir, fileInfo.Name()))
 		if err != nil {
 			continue
 		}