* libc/include/stdlib.h: Add on_exit prototype.
* libc/include/sys/reent.h (struct _atexit): Add argument array
and bits to track type of exit routine to support both on_exit
and atexit.
(_REENT_INIT_PTR): Add missing fields that won't be zeroed out
by default and change the setting of the atexit structure.
(_REENT_INIT)[!_REENT_SMALL]: Remove extraneous end brace.
* libc/stdlib/on_exit.c: New file.
* libc/stdlib/Makefile.am: Add support for on_exit.
* libc/stdlib/Makefile.in: Regenerated.
* libc/stdlib/atexit.c: Change to initialize types field.
* libc/stdlib/exit.c: Change to look at types field for each
exit routine and either call an atexit-style or an on_exit-style
routine accordingly.
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