Create FT_FCWWN field type.

Also, convert the "string" hf_ entries that used tvb_fcwwn_to_str as a string to use proto_tree_add_item with FT_FCWWN type.

Change-Id: I4ca77870499fd8239584a70874998b5d194a7167
Reviewed-on: https://code.wireshark.org/review/6036
Petri-Dish: Michael Mann <mmann78@netscape.net>
Reviewed-by: Michael Mann <mmann78@netscape.net>