Fix (hopefully) scanf behavior for nul bytes in the stream when processing

Fix (hopefully) scanf behavior for nul bytes in the stream when processing
%c, %s, and %[ specifiers.  Note that scanf is undergoing rewrite so I
didn't bother optimizing this.  I did run all my regression tests though.
Set EOF correctly for fmemopen on readonly streams.  I really need to
check what glibc behavior is for the various open modes though.