Update setvbuf to latest OpenBSD implementation

Update setvbuf to latest OpenBSD implementation

Newlib's setvbuf function is very old and has two bugs:
- It sets the SRD/SWR flags incorrectly in case of files opened for
  reading and writing.
  See https://cygwin.com/ml/cygwin/2016-03/msg00180.html
  for a desription of the effect.
- It always sets the buffer size to BUFSIZ if it's not provided by
  the application, independent of the optimal blocksize for the
  underlying IO device.

Update setvbuf to latest code from OpenBSD to fix both problems.

	* libc/stdio/setvbuf.c (setvbuf): Import latest OpenBSD
	implementation.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>