Allows DHCP to work when a bus daemon isn't running.  This also
fixes a race condition where when multiple interfaces are attempting
to get a DHCP lease at the same time, if one DHCP client instance
triggers the callout, that instance gets the bus name, and any
other client triggering the callout at that time will fail because
the bus name is already taken.  Since this commit allows using
a private socket, where no process has a bus name, this race is
avoided.

Also move the DHCP helper from callouts/ to src/dhcp-manager/ to
consolidate all the DHCP stuff and clean up some of the helper's
code.