libcontainer: Setup standard /dev symlinks

When a new mount namespace is initialized, `/dev/fd`, `/dev/stdin`, `/dev/stdout`, and `/dev/stderr` should be set up as well.

Open questions:

1. The list of default dev symlinks is currently package local; should it be exported?
2. Should a `/dev/kcore` symlink also be set up? `systemd` does, `lxc` doesn't.

Fixes #2356
Fixes #5510