- Unpin bootvar and use register ~argv:no_argv` instead.
- Use new name for uplink device ("0", not "tap0").
- Don't configure logging - mirage does that for us now.
Added explicit NAT target, allowing NAT even within client net and
making it clear that NAT is used externally.
Changed Redirect_to_netvm to NAT_to, and allow specifying any target
host.