Make tmpfiles respect configured group name and document security implications

[jtojnar]

Summary

Follow up to #442 (comment).

• Make tmpfiles respect configured polkit group name
• Document the security implications for tmpfiles.d entry

Detailed description and/or reproducer

Make tmpfiles respect configured polkit group name

We allow changing the polkit daemon user through the -Dpolkitd_user Meson option.
When this is used, the tmpfiles.d config will refer to a non-existent group.
For example, NixOS builds polkit with -Dpolkitd_user=polkituser.

Let’s substitute the polkitd_user value into the config.

We do not have a separate option for the group name since, by convention,
the group name matches the user name. This is also enforced by our sysusers.d config:
https://www.freedesktop.org/software/systemd/man/latest/sysusers.d.html#u

Document the security implications for tmpfiles.d entry

Quoting from https://www.github.com/polkit-org/polkit/pull/442#issuecomment-2692718825:

The tmpfiles.d is used to create polkit subdirectory in /etc, because the directory needs specific ownership mode, it needs to be owned by root and only readable by root and polkitd group. This is for security reasons, so that polkit daemon cannot change .rules files in case of getting hijacked. It should have as minimal privileges as possible. The subdirectory is pre-created so no administrator omits to set the appropriate mode and ownership.