journald: introduce new "systemd-journal" group and make it own the journal files

Previously all journal files were owned by "adm". In order to allow
specific users to read the journal files without granting it access to
the full "adm" powers, introduce a new specific group for this.

"systemd-journal" has to be created by the packaging scripts manually at
installation time. It's a good idea to assign a static UID/GID to this
group, since /var/log/journal might be shared across machines via NFS.

This commit also grants read access to the journal files by default to
members of the "wheel" and "adm" groups via file system ACLs, since
these "almost-root" groups should be able to see what's going on on the
system. These ACLs are created by "make install". Packagers probably
need to duplicate this logic in their postinst scripts.

This also adds documentation how to grant access to the journal to
additional users or groups via fs ACLs.

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