Pull request: dhcpd: do not override ra-slaac settings

Merge in DNS/adguard-home from 2653-ra-slaac to master

Updates #2653.

Squashed commit of the following:

commit f261413a58dc813e37cc848606ed490b8c0ac9f3
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Feb 11 20:37:13 2021 +0300

    all: doc changes, rm debug

commit 4a8c6e4897579493c1ca242fb8f0f440c3b51a74
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Feb 11 20:11:46 2021 +0300

    dhcpd: do not override ra-slaac settings

internal/sysutil/net_linux.go

@@ -33,15 +33,19 @@ func ifaceHasStaticIP(ifaceName string) (has bool, err error) {
 		filePath: "/etc/network/interfaces",
 	}} {
 		f, err = os.Open(check.filePath)
-		if errors.Is(err, os.ErrNotExist) {
-			continue
-		}
 		if err != nil {
+			if errors.Is(err, os.ErrNotExist) {
+				err = nil
+
+				continue
+			}
+
 			return false, err
 		}
 		defer f.Close()
 
-		fileReadCloser, err := aghio.LimitReadCloser(f, maxConfigFileSize)
+		var fileReadCloser io.ReadCloser
+		fileReadCloser, err = aghio.LimitReadCloser(f, maxConfigFileSize)
 		if err != nil {
 			return false, err
 		}