Next patches will create transport sockets in task helpers.
As helpers are forked using CLONE_FILES, they must resolve
shared fds to create their own service fds. This patch allows
that.

I've digged in the code, and there is no a reason, we need
pid_rst_prio() during choosing of fdt restorer. So, this
case may be safely deleted, which guarantees, that in case
of TASK_HELPER, the restorer of fdt will be parent, i.e.,
no one TASK_HELPER will be restorer of fdt.

v5: New
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>