I believe the functionalities (both update and deletion) are done in
this patch.  I have not added any tests nor updates on documentation, as
I am not sure what and where they should be.

I have done some basic tests locally to confirm that it works:
- update subfile in a zip archive named "z"
- delete subfile in a zip archive named "z"
- update subfile in a zip archive named "x.zip"
- delete subfile in a zip archive named "x.zip"

FTR, if this is of any importance: the filesystem containing the
archives "z" and "x.zip" is btrfs, and /tmp is ext2 on zram.  Not sure
if any problems would arise in other filesystems / OSes -- hopefully
someone else on those platforms can confirm that it works.

Comments welcome.  Particularly on my choices of naming.