MFC	r268924: Update fflush(3) to return success on a read-only stream.
MFC	r268924:
Update fflush(3) to return success on a read-only stream.

This is done for compliance with SUSv3. The changes cause
no secondary effects in the gnulib tests (we pass them).

Obtained from:	Apple Inc. (Libc 997.90.3 with changes)
Reviewed by:	bde
Phabric:	D440

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