forking: Always close pid proc before child creation

Child does not know about parent's pid proc fd,
and it can't close it by fd. Next patch will do
close_old_files() optional, and it will base on
the fact there is no leftover fds. So, close pid
proc directly.

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