Register _cleanup function with atexit

 * Register cleanup function with atexit
   instead of calling it explicitly on
   exit()
 * abort() no longer calls _cleanup:
   Flushing stdio buffers on abort is no
   longer required by POSIX.
 * dlmalloc no longer need to reset cleanup
   (see above)
 * Upstream findfp.c makebuf.c setvbuf.cexit.c
   to openbsd versions.

Bug: 14415367
Change-Id: I277058852485a9d3dbb13e5c232db5f9948d78ac