* try to use "/usr/sbin/ip" before "ifconfig" since the latter is
obsolete on Linux to figure out interface to listen for DHCP messages.
But keeping ifconfig method just in case.
Without this, tests fail on boxes without ifconfig AND the correct
interface named ethX.
* amend possible failure responses, as check_dhcp might get a DHCPOFFER,
but from an unexpected address - which happens if the box running
tests are on a network with a DHCP server.
