(jka-compr-info-file-magic-bytes): New function.

(jka-compr-info-file-magic-bytes): New function.
(jka-compr-compression-info-list): Add new elt to each vector.
(jka-compr-write-region): Don't compress the data if it is already compressed.

(jka-compr-really-do-compress): New variable.
(jka-compr-insert-file-contents): Set jka-compr-really-do-compress if visiting.
(jka-compr-write-region): Set jka-compr-really-do-compress
if visiting.  Test it when deciding to compress.