Decompress buffer's contents, depending on jka-compr.
Only when FORCE is t or `auto-compression-mode' is enabled and FILENAME
agrees with `jka-compr-compression-info-list', decompression is done.
Signal an error if FORCE is neither nil nor t and compressed data are
not decompressed because `auto-compression-mode' is disabled.
If INPLACE is nil, return decompressed data or nil without modifying
the buffer. Otherwise, replace the buffer's contents with the
decompressed data. The buffer's multibyteness must be turned off.