When given -c -a, issue a warning if no compressor is associated with the suffix.

When given -c -a, issue a warning if no compressor is associated with the suffix.

* src/suffix.c (find_compression_suffix): Always return stripped
archive name length in the last argument.  Return 0 if there is no
suffix.
(find_compression_program): Remove.
(set_compression_program_by_suffix): Take third argument, controlling
whether to issue a warning if no suitable compression program is found
for the suffix.
* src/common.h (set_compression_program_by_suffix): Change prototype.
* src/buffer.c, src/tar.c: All uses of set_compression_program_by_suffix
changed.