popen: allow to kill process group

As Alexander pointed out this might be useful
for running a pipe of programs inside shell
(i.e. popen.shell('foo | bar | baz', 'r')).

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>