rump: Add ISO9660 support
run: Add 'check_installed' command to run env

'check_installed' takes a command name as argument and tries to call 'which' in
order to find the command path. If that does not succeed, paths like '/sbin' are
'/usr/sbin' searched. On success the absolute path of the command is returned,
on failure 'exit' is called with an error message.
rump: Added tool check to run scripts