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/sql_string.h

@@ -575,6 +575,10 @@ public:
   {
     qs_append(str, (uint32)strlen(str));
   }
+  void qs_append(const LEX_CSTRING *str)
+  {
+    qs_append(str->str, str->length);
+  }
   void qs_append(const char *str, uint32 len);
   void qs_append_hex(const char *str, uint32 len);
   void qs_append(double d);