Read file FILENAME into a buffer and return the buffer.
If a buffer exists visiting FILENAME, return that one, but
verify that the file has not changed since visited or saved.
The buffer is not selected, just returned to the caller.
Optional second arg NOWARN non-nil means suppress any warning messages.
Optional third arg RAWFILE non-nil means the file is read literally.
Optional fourth arg WILDCARDS non-nil means do wildcard processing
and visit all the matching files. When wildcards are actually
used and expanded, return a list of buffers that are visiting the various files.