popen: add ability to keep child on deletion

Currently popen_delete kills all children process.
Moreover we use popen_delete on tarantool exit.

Alexander pointed out that keep children running
even if tarantool is exited is still needed.

Part of #4031

Reported-by: Alexander Turenko <alexander.turenko@tarantool.org>
Acked-by: Alexander Turenko <alexander.turenko@tarantool.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>