New APIs for embedding applications that want to add their own entries

New APIs for embedding applications that want to add their own entries
to the table of built-in modules.  This should normally be called
*before* Py_Initialize().  When the malloc() or realloc() call fails,
-1 is returned and the existing table is unchanged.

After a similar function by Just van Rossum.

int PyImport_ExtendInittab(struct _inittab *newtab);
int PyImport_AppendInittab(char *name, void (*initfunc)());