useradd: undocumented /usr/etc/skel support

[ldv-alt]

PR #591 introduced /usr/etc/skel support in useradd without any mentioning in the documentation whatsoever.
According to the implementation, useradd populates the home directory by merging contents of /etc/skel and /usr/etc/skel.
Also, unlike /etc/skel, /usr/etc/skel is being used regardless of -k/--skel option.

Why /usr/etc/skel support was implemented this way is unclear: commit 74c17c7 references a specification that doesn't exist, and the specification that does exist (https://uapi-group.org/specifications/specs/extension_image/) doesn't seem to be relevant.

Anyway, this undocumented feature creates uncertainty. We've got a linux-pam/linux-pam#920 against pam_mkhomedir to implement this undocumented feature in Linux-PAM, but I'm reluctant to do so because of the issues mentioned above.

Please clarify.

[ikerexxe]

@jubalh @hallyn can you take a look? I didn't follow the original PR so I think best if you could take a look at this issue.

[hallyn]

Found an old copy of the page here: https://web.archive.org/web/20221026182047/https://uapi-group.org/specifications/specs/base_directory_specification/

And so based on that, the new location appears to be:

https://specifications.freedesktop.org/basedir-spec/latest/