Added bitmap_buffer_size()

Removed valgrind warnings when using not aligned bitmap buffer size
Added setting of thread_stack


include/my_bitmap.h:
  Added bitmap_buffer_size()
sql/handler.cc:
  use bitmap_buffer_size()
sql/opt_range.cc:
  use bitmap_buffer_size()
sql/sql_insert.cc:
  Use bitmap_buffer_size()
sql/sql_parse.cc:
  Added setting of thread_stack when taking thread from thread cache
sql/sql_plugin.cc:
  Set thread_stack

sql/sql_parse.cc

@@ -1176,6 +1176,7 @@ end_thread:
       or this thread has been schedule to handle the next query
     */
     thd= current_thd;
+    thd->thread_stack= (char*) &thd;
   } while (!(test_flags & TEST_NO_THREADS));
   /* The following is only executed if we are not using --one-thread */
   return(0);					/* purecov: deadcode */