enable hiding of extended attributes in client configuration via CVMFS_HIDE_MAGIC_XATTRS

[boegel]

The extended attributes used by CernVM-FS cause trouble in some scenarios, for example when files are being copied from the mounted CVMFS repository into another type of filesystem (see for example easybuilders/easybuild-framework#3910).

That can be avoided by enabling CVMFS_HIDE_MAGIC_XATTRS in the client configuration, so the extended attributes are not exposed on the client.

[bedroge]

@huebner-m ran into issues with our build container, lots of file operations were returning No space left on device. I also encountered this myself when trying to overwrite some files in the compatibility layer inside the build container. After some debugging I found out that it only happens with CVMFS 2.9.x, which reminded me of the issue with the additional extended attributes. Setting CVMFS_HIDE_MAGIC_XATTRS=yes in default.local of the build container solves the issue, so I'll make a PR to change this.

[bedroge]

See #125.

A newer version of fuse-overlayfs could possibly solve the issue as well, in the release notes of 1.5.0 (see https://github.com/containers/fuse-overlayfs/releases/tag/v1.5.0) I found fix copyup of xattrs longer than 256 bytes. .

[boegel]

As discussed in #125 (comment), we should also document this in the [recommended configuration in the README](https://github.com/EESSI/filesystem-layer/blame/main/README.md#L183)