Pull request: all: support setgid, setuid on unix

Updates #2763.

Squashed commit of the following:

commit bd2077c6569b53ae341a58aa73de6063d7037e8e
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Fri Jun 4 16:25:17 2021 +0300

    all: move rlimit_nofile, imp docs

commit ba95d4ab7c722bf83300d626a598aface37539ad
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Fri Jun 4 15:12:23 2021 +0300

    all: support setgid, setuid on unix

HACKING.md

@@ -126,6 +126,9 @@ on GitHub and most other Markdown renderers. -->
     )
     ```
 
+ *  Don't rely only on file names for build tags to work.  Always add build tags
+    as well.
+
  *  Don't use `fmt.Sprintf` where a more structured approach to string
     conversion could be used.  For example, `net.JoinHostPort` or
     `url.(*URL).String`.