Add support for a --device flag. Refactor device handling code

This is try number ? for https://github.com/dotcloud/docker/pull/4191

We add a `--device` flag that can be used for adding devices to containers.  We create a global list of devices that are allowed in containers, and devices that are to be created in containers.  We fix inconsistencies between the device handling of the lxc exec driver, native-fs exec driver and the native-systemd exec drivers.