provide more information to the pre/postRPMSign callback
    
    The existing pre/postRPMSign callback was following the format of
    the Task/BuildStateChange callbacks. This isn't necessary for a
    one-time operation. This change removes the attribute/old/new
    arguments, and replaces them with sigkey, sighash, build, and rpm
    arguments. This is more clear, and should make writing handlers
    for this callback simpler.