added dp_desc field to ofp_stats_desc

added a human readable string to ofp_stats_desc to describe
the datapath, e.g., "switch4 in closet 3b of Gates"

* updated openflow.h
* updated udatapath: defaults to "$HOSTNAME pid=$$"
* updated kernel datapath: defaults to "$HOSTNAME idx=$i"
* updated wireshark plugin
* added regression test for ofp_desc_stats
* addressed KK's review comments from:
    https://mailman.stanford.edu/pipermail/openflow-dev/2009-December/000545.html

include/openflow/openflow.h

@@ -729,8 +729,9 @@ struct ofp_desc_stats {
     char hw_desc[DESC_STR_LEN];        /* Hardware description. */
     char sw_desc[DESC_STR_LEN];        /* Software description. */
     char serial_num[SERIAL_NUM_LEN];   /* Serial number. */
+    char dp_desc[DESC_STR_LEN];        /* Human readable description of datapath. */
 };
-OFP_ASSERT(sizeof(struct ofp_desc_stats) == 800);
+OFP_ASSERT(sizeof(struct ofp_desc_stats) == 1056);
 
 /* Body for ofp_stats_request of type OFPST_FLOW. */
 struct ofp_flow_stats_request {