Pull request: 3257 source directive

Merge in DNS/adguard-home from 3257-ifaces-source to master

Updates #3257.

Squashed commit of the following:

commit 0b9b42bab731bbd048e97893cf209794ea014dfe
Merge: 530a1a23 e25a5329
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Mon Jun 28 16:53:36 2021 +0300

    Merge branch 'master' into 3257-ifaces-source

commit 530a1a23a601c5575c8dc5f6f97cd84801cf911b
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Jun 25 19:43:55 2021 +0300

    aghnet: imp code, add docs

commit 58de84821b93bcbb3df1834ba33fbad817201b1d
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Jun 25 13:34:43 2021 +0300

    aghnet: sup "source" directive

commit c0901abd5212902295e8ee546fad652092fdb5a8
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Thu Jun 24 16:46:03 2021 +0300

    aghos: mv func to aghnet

HACKING.md

@@ -98,8 +98,9 @@ attributes to make it work in Markdown renderers that strip "id". -->
  *  Constructors should validate their arguments and return meaningful errors.
     As a corollary, avoid lazy initialization.
 
- *  Define `MarshalFoo` methods on non-pointer receivers, as pointer receivers
-    [can have surprising results][staticcheck-911].
+ *  Prefer to define methods on pointer receievers, unless the type is small or
+    a non-pointer receiever is required, for example `MarshalFoo` methods (see
+    [staticcheck-911]).
 
  *  Don't mix horizontal and vertical placement of arguments in function and
     method calls.  That is, either this: