files: Implement find_unused_file_desc_id()

This function will be used to allocate id for fake files
(don't confuse with fake fds, e.g. fles).

Suggested-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>