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/os_windows.go

@@ -5,7 +5,6 @@
 package aghos
 
 import (
-	"fmt"
 	"syscall"
 
 	"golang.org/x/sys/windows"
@@ -16,7 +15,7 @@ func canBindPrivilegedPorts() (can bool, err error) {
 }
 
 func setRlimit(val uint64) (err error) {
-	return ErrUnsupported
+	return Unsupported("setrlimit")
 }
 
 func haveAdminRights() (bool, error) {
@@ -41,7 +40,7 @@ func haveAdminRights() (bool, error) {
 }
 
 func sendProcessSignal(pid int, sig syscall.Signal) error {
-	return fmt.Errorf("not supported on Windows")
+	return Unsupported("kill")
 }
 
 func isOpenWrt() (ok bool) {