use one 'store' method to rule all (de)compressors

use one 'store' method to rule all (de)compressors

Adding a new compressor method meant adding a new method as well – even
if that boilt down to just linking to our generalized decompressor with
a new name. That is unneeded busywork if we can instead just call the
generalized decompressor and let it figure out which compressor to use
based on the filenames rather than by program name.

For compatibility we ship still 'gzip', 'bzip2' and co, but they are
just links to our "new" 'store' method.