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_unix.go

@@ -0,0 +1,50 @@
+// +build darwin freebsd linux netbsd openbsd
+
+//go:build darwin || freebsd || linux || netbsd || openbsd
+
+package aghos
+
+import (
+	"fmt"
+	"os/user"
+	"strconv"
+	"syscall"
+)
+
+func setGroup(groupName string) (err error) {
+	g, err := user.LookupGroup(groupName)
+	if err != nil {
+		return fmt.Errorf("looking up group: %w", err)
+	}
+
+	gid, err := strconv.Atoi(g.Gid)
+	if err != nil {
+		return fmt.Errorf("parsing gid: %w", err)
+	}
+
+	err = syscall.Setgid(gid)
+	if err != nil {
+		return fmt.Errorf("setting gid: %w", err)
+	}
+
+	return nil
+}
+
+func setUser(userName string) (err error) {
+	u, err := user.Lookup(userName)
+	if err != nil {
+		return fmt.Errorf("looking up user: %w", err)
+	}
+
+	uid, err := strconv.Atoi(u.Uid)
+	if err != nil {
+		return fmt.Errorf("parsing uid: %w", err)
+	}
+
+	err = syscall.Setuid(uid)
+	if err != nil {
+		return fmt.Errorf("setting uid: %w", err)
+	}
+
+	return nil
+}