Before this patch we used flock to order task creation,
but this way is not good. It took 5 syscalls to synchronize
a creation of a single child:

1)open()
2)flock(LOCK_EX)
3)flock(LOCK_UN)
4)close() in parent
5)close() in child

The patch introduces more effective way for synchronization,
which executes 2 syscalls only. We use last_pid_mutex,
and the syscalls number sounds definitely better.

v2: Don't use flock() at all
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>