Lots of small cleanups

- Simplified use_trans_cache() to return at once if is_transactional is set
- Indentation and spelling errors fixed
- Don't call signal_update() if update_binlog_end_pos() is called as the
  function already calls signal_update()
- Removed not used function wait_for_update_bin_log(), which would cause
  errors if ever used.
- Simplified handler::clone() by always allocating 'ref' in ha_open(). To do
  this I added an optional MEM_ROOT argument to ha_open() to be used when
  allocating 'ref'
- Changed arguments to get_system_var() from LEX_CSTRING to LEX_CSTRING*
- Added THD as argument to create_select_for_variable(). Changed also char*
  argument to LEX_CSTRING to avoid strlen() call.
- Change calls to append() to use LEX_CSTRING

sql/handler.h

@@ -2832,7 +2832,8 @@ public:
   }
   /* ha_ methods: pubilc wrappers for private virtual API */
   
-  int ha_open(TABLE *table, const char *name, int mode, uint test_if_locked);
+  int ha_open(TABLE *table, const char *name, int mode, uint test_if_locked,
+              MEM_ROOT *mem_root= 0);
   int ha_index_init(uint idx, bool sorted)
   {
     DBUG_EXECUTE_IF("ha_index_init_fail", return HA_ERR_TABLE_DEF_CHANGED;);