Feature: fencer: track full result in fencing history

Add fencing operation results when creating XML in
stonith_local_history_diff_and_merge(), and parse the results from the received
XML in stonith_xml_history_to_list().

With this, the fencer now always has full results in its op list, and returns
them in the reply for STONITH_OP_FENCE_HISTORY requests (though nothing uses
that as of this commit).