Fixes after comments from last push:

- Removed some not needed casts
- Change plugin.h to be 'binary compatible' with old versions
- Added mysql_ft_size_t typedef to plugin.h to make it trivial to change string lengths to size_t on next ABI change
- Made some fixes suggested by Kristian to make things more portable and future safe (when it comes to strict aliasing)

include/ft_global.h:
  Introduced FT_WEIGTH, to handle fulltext weights in a slightly more portable manner
include/mysql/plugin.h:
  Change plugin.h to be 'binary compatible' with old versions
  Added mysql_ft_size_t typedef to plugin.h to make it trivial to change string lengths to size_t on next ABI change
  Changed flags to unsigned (as flags should always be unsigned)
mysql-test/t/information_schema.test:
  Fixed typo
sql/sp_head.cc:
  Removed cast
sql/sql_select.cc:
  Removed cast
sql/table.cc:
  Removed cast
storage/maria/ma_ft_boolean_search.c:
  Use mysql_ft_size_t instead of size_t for plugin.h code
  Changed some other string lengths to size_t
storage/maria/ma_ft_nlq_search.c:
  Use FT_WEIGTH to make code more portable
storage/maria/ma_ft_parser.c:
  Use mysql_ft_size_t instead of size_t for plugin.h code
  Changed some other string lengths to size_t
storage/maria/ma_ftdefs.h:
  Changed some string lengths to size_t
storage/maria/maria_ftdump.c:
  Use FT_WEIGTH to make code more portable
storage/myisam/ft_boolean_search.c:
  Use mysql_ft_size_t instead of size_t for plugin.h code
storage/myisam/ft_nlq_search.c:
  Use FT_WEIGTH to make code more portable
storage/myisam/ft_parser.c:
  Use mysql_ft_size_t instead of size_t for plugin.h code
storage/myisam/myisam_ftdump.c:
  Use FT_WEIGTH to make code more portable

sql/sql_select.cc

@@ -3586,7 +3586,7 @@ add_key_fields(JOIN *join, KEY_FIELD **key_fields, uint *and_level,
         {
           if (!field->eq(item->field))
           {
-            Item *tmp_item= (Item*) item;
+            Item *tmp_item= item;
             add_key_field(key_fields, *and_level, cond_func, field,
                           TRUE, &tmp_item, 1, usable_tables,
                           sargables);