Add is_utf8_fixed_width_buf_flags() and use it

Add is_utf8_fixed_width_buf_flags() and use it

This encodes a simple pattern that may not be immediately obvious to
someone needing it.  If you have a fixed-size buffer that is full of
purportedly UTF-8 bytes, is it valid or not?  It's easy to do, as shown
in this commit.  The file test operators -T and -B can be simpified by
using this function.