Support adding DHCP interfaces one at a time.

Refactors dhcp-all-interfaces.sh so that if an optional
INTERFACE argument (the first argument) is passed to the script
it only inspects that single interface. If no argument is
passed then the previous default behaviour is used which
causes all interfaces to be inspected.

To avoid a collision with the previous $1 we move to using
$FLOCKED for the exec flock command which runs on distributions
using ENI.

Also sets PATH so that the commands within the script
can all be found if it isn't set properly (/sbin/ip, /bin/cat, etc.)

This is a move towards using udev rules to add these types
of interfaces automatically.

Change-Id: I3ec8fd2cc2071bfc6943c744ca619e31b71146fc
Merge "Support adding DHCP interfaces one at a time."