eosfs: use external script for home creation + drop support for shadow namespace

[glpatcern]

This comes as part of the effort to operate EOS without being root, see #4977

In this PR the post-home creation hook (and corresponding flag) is replaced by a create_home_hook, and the following configuration parameters are suppressed:

• shadow_namespace
• share_folder
• default_quota_bytes
• default_secondary_quota_bytes
• default_quota_files
• uploads_namespace (unused)

[update-docs]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would create a changelog item based on your changes.

[diocas]

What was the reason for the shadow ns? Maybe it's worth to explain in the PR why it's ok to remove it.

[glpatcern]

What was the reason for the shadow ns? Maybe it's worth to explain in the PR why it's ok to remove it.

I actually put that in a ADR. Hugo just confirmed this is a legacy attempt to implement the "share jail", but it has never been used.