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

internal/aghos/user_windows.go

@@ -0,0 +1,16 @@
+// +build windows
+
+//go:build windows
+
+package aghos
+
+// TODO(a.garipov): Think of a way to implement these.  Perhaps by using
+// syscall.CreateProcessAsUser or something from the golang.org/x/sys module.
+
+func setGroup(_ string) (err error) {
+	return Unsupported("setgid")
+}
+
+func setUser(_ string) (err error) {
+	return Unsupported("setuid")
+}