restore: handle errors of setting credentials (v2)

v2: handle errors from setXids and securebits manipulations
    handle errors of restoring creds after finishing CR_STATE_RESTORE_CREDS,
    because a sigchild handler is already restored in this moment.
    Only the current process is killed in a error case.

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