Add "lxc enter" from "pventer" without its deps.

This patch adds the functionality of "pventer" to "pveclient" without
its external dependecies, we solved them with our internal tools.

I tried to use IO::Select->select() but failed hard with it, so i
switched back to $select->can_(read|write).

Signed-off-by: René Jochum <r.jochum@proxmox.com>