* malloc_wrapper.cc: Change 'use_internal_malloc' to 'use_internal' throughout.

* malloc_wrapper.cc: Change 'use_internal_malloc' to 'use_internal' throughout.
(export_malloc_called): Delete.
(internal_malloc_determined): New variable.
(malloc_init): Control calculation of internal/external malloc based on
'internal_malloc_determined'.  Use import_address() to determine if malloc in
user_data is ours or not.
* miscfuncs.cc (thread_wrapper): Make static.
(__import_address): Define new function.
* miscfuncs.h (import_address): New define.
(__import_address): Declare new function.