Changed existing interface; see commit
219324e6 about the refactoring.

The error seen was:

[ RUN      ] PerfPkt4Test.RawUnpack
/libexec/ld-elf.so.1: /usr/home/jreed/builder/work/BIND10-cppcheck/20120620073005-FreeBSD8-amd64-GCC/build/tests/tools/perfdhcp/tests/.libs/run_unittests: Undefined symbol "_ZN3isc4dhcp6Option7setDataEN9__gnu_cxx17__normal_iteratorIPKhSt6vectorIhSaIhEEEES9_"