Pull request: 3020 runtime clients sources control

Merge in DNS/adguard-home from 3020-client-sources to master

Closes #3020.

Squashed commit of the following:

commit f8e6b6d63373f99b52f7b8c32f4242c453daf1a4
Merge: 41fb071d 0a1ff65b
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon Apr 25 19:19:15 2022 +0300

    Merge branch 'master' into 3020-client-sources

commit 41fb071deb2a87e0a69d09c8f418a016b4dd7e93
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Mon Apr 25 13:38:28 2022 +0300

    home: fix nil, imp docs

commit aaa7765914a8a4645eba357cd088a9470611ffdc
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Mon Apr 25 12:25:47 2022 +0300

    home: imp code

commit 3f71b999564c604583b46313d29f5b70cf51ee14
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Fri Apr 22 19:12:27 2022 +0300

    home: runtime clients sources control

internal/home/options.go

@@ -230,13 +230,19 @@ var helpArg = arg{
 }
 
 var noEtcHostsArg = arg{
-	description:     "Do not use the OS-provided hosts.",
+	description:     "Deprecated.  Do not use the OS-provided hosts.",
 	longName:        "no-etc-hosts",
 	shortName:       "",
 	updateWithValue: nil,
 	updateNoValue:   func(o options) (options, error) { o.noEtcHosts = true; return o, nil },
-	effect:          nil,
-	serialize:       func(o options) []string { return boolSliceOrNil(o.noEtcHosts) },
+	effect: func(_ options, _ string) (f effect, err error) {
+		log.Info(
+			"warning: --no-etc-hosts flag is deprecated and will be removed in the future versions",
+		)
+
+		return nil, nil
+	},
+	serialize: func(o options) []string { return boolSliceOrNil(o.noEtcHosts) },
 }
 
 var localFrontendArg = arg{