parasite: return ability to execute code in a thread with specified pid

Currently we use id to get pid and other properties, which used when a
thread is daemonized. We are going to not demonize threads, so pid will
be only one useful field on parasite_thread_ctl.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>