cr-restore: Open transport socket earlier

I need named socket to communicate with pid_ns helpers
(see next patches) and receive answer from them
(it's impossible to send answer to unnamed socket).
As we already have transport socket, we'll reuse it
for the above goal too.

This patch makes transport sockets be created before
creation of children tasks. Also, now they are created
not only for alive tasks (so we need additional
manipulations for TASK_HELPERS, e.g., to call prepare_fdt()).

v5: Return CLONE_FILES clone() argument during task helpers
creation. Also get rid of fdt_mutex as CLONE_FILES processes
does not close old files after clone, and we don't have
intertersections between them. Also, socket() system call
can't return a fd in service fds range, which was the main
reason to have this mutex.

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