libnetutils: Fix up the dhcp request arguments

The interface/config-file arguments passed to the dhcpcd invocation
when a hostname isn't provided were swapped, causing requests to be made
for interface <config-file-name> instead of <interface-name>, and
failing all calls to start dhcpcd through netutils with the following error:

E/DhcpStateMachine( 3214): DHCP failed on wlan0: Timed out waiting for dhcpcd to start

because...

<3>[  582.717015] init: no such service 'dhcpcd_/system/etc/dhcpcd/dhcpcd.conf:-f wlan0 wlan0'

Change-Id: Ic2e026ae8ebebdc9a27f5d7a6274e1c5c8a2f95e