FreeBSD sysctl does not include the "hw.physicalcpu" OID

FreeBSD does not seem to support "hw.physicalcpu", so it seems reasonable to remove it from Parallel.physical_processor_count(). Attempting to access it just raises an error on FreeBSD 9.

    # uname -mor
    FreeBSD 9.1-RELEASE amd64
    # sysctl -n hw.physicalcpu
    sysctl: unknown oid 'hw.physicalcpu'