Pull request:* all: fix all staticcheck SA warnings

Merge in DNS/adguard-home from 2238-fix-static-analisys-warnings to master

Squashed commit of the following:

commit 721ca6fa1cbfdfe9d414e6ed52fec4a64653fb52
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Oct 30 15:48:10 2020 +0300

    * all: fix all staticcheck SA warnings

    Closes #2238.

internal/dhcpd/v6.go

@@ -45,6 +45,7 @@ func ip6InRange(start net.IP, ip net.IP) bool {
 	if len(start) != 16 {
 		return false
 	}
+	//lint:ignore SA1021 TODO(e.burkov): Ignore this for now, think about using masks.
 	if !bytes.Equal(start[:15], ip[:15]) {
 		return false
 	}