Add _openslide_fread_exact()

Many _openslide_fread() callers want to fail unless the specified number
of bytes is read.  Add a helper for this.

Stop detecting TOCTTOU file size increases in _openslide_read_key_file().
They're harmless to us, and input files are expected to be read-only.

Signed-off-by: Benjamin Gilbert <bgilbert@cs.cmu.edu>