Fix ifquery call in dhcp-all-interfaces

Ifquery does not print anything for interfaces which only have a single
iface line. It does, however, return non-zero if the interface is not
configured at all, so we can use that to indicate whether or not there
is a configuration.

Fixes bug #1233579

Change-Id: Ia2fdafbea57e806eba99ae8ddaf395ebdcc306e1