Support field clearing when the provided source buffer is null.
Inital DHCP parsing.

- DHCP Packet header and fields are defined.

- The parser now supports parsing based on L4 port information.
  The PPE parsable service port list defines the ports currently recognized.
    Any value added to this list must have a parse function.
    DHCP is listed as a recognized service port.
Merge into master from pull request #8:
Initial DHCP parsing support.  (https://github.com/floodlight/bigcode/pull/8)