builtin port forwarder resource leak

[AkihiroSuda]

$ rootlesskit --state-dir=/tmp/foo -p 127.0.0.1:8000:8000/tcp --net=slirp4netns --port-driver=builtin python3 -m http.server

$ ls /proc/$(cat /tmp/foo/child_pid)/fd | wc -l
7
$ curl -s http://localhost:8000 > /dev/null
$ ls /proc/$(cat /tmp/foo/child_pid)/fd | wc -l
9
$ curl -s http://localhost:8000 > /dev/null
$ ls /proc/$(cat /tmp/foo/child_pid)/fd | wc -l
11