unbreak the copy-method claiming hashsum mismatch since ~exp9

unbreak the copy-method claiming hashsum mismatch since ~exp9

Commit 653ef26c70dc9c0e2cbfdd4e79117876bb63e87d broke the camels back in
sofar that everything works in terms of our internal use of copy:/, but
external use is completely destroyed. This is kinda the reverse of what
happened in "parallel" in the sid branch, where external use was mostly
fine, internal and external exploded on the GzipIndexes option.

We fix this now by rewriting our internal use by letting copy:/ only do
what the name suggests it does: Copy files and not uncompress them
on-the-fly. Then we teach copy and the uncompressors how to deal with
/dev/null and use it as destination file in case we don't want to store
the uncompressed files on disk.

Closes: 799158