Run alternative decompression programs if the principal one is not available.

Run alternative decompression programs if the principal one is not available.

Some compression programs are able to handle various compression formats
(e.g. `gzip' can decompress files created by `compress', `xz' is able
to handle lzma, etc.)  Tar tries to use such programs for decompression
if the principal decompressor cannot be started.

* src/buffer.c (compress_type): Swap ct_none and ct_tar.
(archive_compression_type): New static variable.
(zip_magic): Remove program and option fields.
(zip_program): New structure and static.
(compress_program): Remove macro.
(find_zip_program): New static function.
(first_decompress_program,next_decompress_program): New functions.
(open_compressed_archive): Set archive_compression_type instead of
use_compress_program_option.
* src/common.h (first_decompress_program)
(next_decompress_program): New functions.
(WARN_DECOMPRESS_PROGRAM): New flag.
(WARN_VERBOSE_WARNINGS): Include WARN_DECOMPRESS_PROGRAM.
* src/warning.c (warning_args): Add "decompress-program".
(warning_types): Add WARN_DECOMPRESS_PROGRAM.
* src/system.c (run_decompress_program): New function.
(sys_child_open_for_uncompress): Use run_decompress_program
instead of calling execlp directly.