Use zsyncfile API, remove duplicated code

[ckamm]

Combines with ahmedammar/zsync#1

Note that this is based on #6382 to allow actual testing. This PR has only one commit: c742b4a

[ogoffart]

CI system is confused:

fatal: reference is not a tree: dde7f5b80d4f8fe0ef3a2959852f0a8ed9cafb5c
Unable to checkout 'dde7f5b80d4f8fe0ef3a2959852f0a8ed9cafb5c' in submodule path 'src/3rdparty/zsync'

Should we maybe put zsync in the owncloud umbrella ?

[ckamm]

I think that's just the pain of using git submodules like that, it wouldn't go away if it was inside the oc organization. I need to update the pointed submodule ref once the PR there has been merged. (which is done)

[ckamm]

Do not merge yet because I need to update the submodule sha1 to the correct one after the zsync PR got merged.

[ckamm]

@ogoffart CI fails with

src/3rdparty/zsync/c/libzsync/zsync.c:44:23: fatal error: arpa/inet.h: No such file or directory

Who do I ask about making this header available?

[ogoffart]

Who do I ask about making this header available?

Modify the admin/linux/DockerFile and update accordingly the docker image in the Jenkinsfile
I gess @dschmidt might be able to help with that.

[ckamm]

@dschmidt I'd add libc6-dev to Dockerfile for this PR. Would you be able to rebuild the image?

[ckamm]

The problem will be resolved by adjusting zsync headers. ahmedammar/zsync#3