Comment 5 for bug 838968

Discussed on IRC, the problem here is that ifupdown invokes dhclient in a way that it still exits with success when it has *failed* to acquire an IP address, causing ifupdown to emit the interface-up event.

Passing -1 to dhclient3 gives the correct behavior: daemonize when an address is acquired, and if the timeout is reached without acquiring an address, exit non-zero instead of daemonizing.

There's a question of whether we should also raise the timeout in dhclient to accomodate systems like the one where this was first encountered, where 60 seconds isn't long enough to get the interface up and get an address via dhcp; but at least the ifupdown change is a per-se-correct one-line fix.