1.0.23.9: extend pa_alloc to accept a page_type_flag

1.0.23.9: extend pa_alloc to accept a page_type_flag

 (Code and data separation 4/5.)

 * Define alloc() and pa_alloc() in terms of a new function,
   general_alloc(), which accepts a page_type_flag.

   Guts of old alloc() are now called general_alloc_internal(), called
   by general_alloc() after selecting the region to use, and locking
   if using a non-thread-local region.