fread: improve performance for unbuffered reads

We can use the buffer passed to fread(3) directly in the FILE *.
The buffer needs to be reset before each call to __srefill().
This preserves the expected behavior in all cases.

The change was found originally in OpenBSD and later adopted by NetBSD.

MFC after: 2 weeks
Obtained from: OpenBSD (CVS 1.18)

Differential Revision: https://reviews.freebsd.org/D30548