add of_str64_t string type

We need a 64-byte string for the debug_counter extension. Since adding a new
string type is so much work, the new type is named generically so that any
extension can use it.
diff --git a/c_gen/of_g_legacy.py b/c_gen/of_g_legacy.py
index 1fadba0..3311282 100644
--- a/c_gen/of_g_legacy.py
+++ b/c_gen/of_g_legacy.py
@@ -192,6 +192,7 @@
                          short_name="desc_str"),
     of_serial_num_t = dict(bytes=ofp_constants["OF_SERIAL_NUM_LEN"],
                            short_name="ser_num"),
+    of_str64_t = dict(bytes=64, short_name="str64"),
     of_match_v1_t = dict(bytes=40, to_w="match_v1_hton",
                          from_w="match_v1_ntoh",
                          short_name="match_v1"),
@@ -213,7 +214,8 @@
                    "of_port_no_t", "of_fm_cmd_t", "of_wc_bmap_t",
                    "of_match_bmap_t", "of_port_name_t", "of_table_name_t",
                    "of_desc_str_t", "of_serial_num_t", "of_mac_addr_t",
-                   "of_ipv6_t", "of_ipv4_t", "of_bitmap_128_t", "of_checksum_128_t"]
+                   "of_ipv6_t", "of_ipv4_t", "of_bitmap_128_t", "of_checksum_128_t",
+                   "of_str64_t"]
 
 ##
 # LOXI identifiers