copy: retire splice use() for copying files on disk

copy: retire splice use() for copying files on disk

Apparently splice() is quite problematic, hence just don't anymore. It's
also unnecessary these days since either copy_file_range() or sendfile()
nowadays typically work, the splice() fallback doesn't give us much
anymore.

(At least I am not aware of a combo of fds where splice() would work
where neither cfr nor sf would work).

This leaves one use of splice() in place, in
src/shared/socket-forward.c. We should probably kill that too, but
that'd require some reworking to use sendfile() I guess, and I am too
lazy for that right now. Moreover, in contrast to the other uses it's
probably even safe, since it uses an intermediary pipe always. But what
do I know...

Fixes: #29044