restore: collect signals from zombies (v4)

Each zombie sends SIGCHLD to parent. crtools restores all pending
signals, so all other signals should be collected.

Here is a problems, that signals SIGCHLD can be merged, but crtools
should be sure, that all signals are collected.

For that a zombie locks a global zombie_lock, which is released by
parent.

This operation should be done between CR_STATE_RESTORE and
CR_STATE_RESTORE_SIGCHLD.

Here is one more CR_STATE_RESTORE_ZOMBIES, whic is used for waiting all
zombies.

v2: clean up
v3: rework synchronization
v4: rework without additional CR_STATE-s

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