T1843: use include files for DHCP/DHCPv6 options
As 219779b ("T1843: run interface-definitions though GCC preprocessor")
implemented the foundation of using the GCC preprocessor to make our XML
definitions more lightweight this commit transforms the configuration of DHCP/
DHCPv6 configuration options to this new style. It implementes it for the
following interface types:
- bonding
- bridge
- ethernet
- wireless
- vif/vif-s interfaces