in-band: Implement L3-based in-band control

in-band: Implement L3-based in-band control

Previously, in-band control was L2-based.  This worked well when the
controller was on the same network segment as the switch.  However, many
configurations are not set up this way.  These changes allow a switch and
controller to be on different subnets.

This set of changes also fixes some problems related to passing DHCP
traffic as described in Bug #1618.

A full description of the reasoning and supported configurations of
in-band will be forthcoming.