multi-window screen, tap switches

- use one multi-window screen instead of one screen per instance

- add /sbin to PATH, which is needed for start-stop-daemon

- removed the root_image parameter from the ubd0 argument because it can't be a full path and is redundant anyway, as the COW file contains the base image filename

- added a tap_switches option, which assigns a tap device to each uml_switch, this way tcpdump can be run from the host without the need of sniffer hosts

- made the rc script work on non-debian systems as well (tried on gentoo, in
that case it goes in /etc/local.d/uml.start)
BTW, -n is a shorter way of writing ! -z