Make taint message structured and add catalog entry

Dec 14 14:10:54 krowka elogind[1]: System is tainted: overflowgid-not-65534
-- Subject: The system is configured in a way that might cause problems
-- Defined-By: elogind
-- Support: https://lists.freedesktop.org/mailman/listinfo/elogind-devel
--
-- The following "tags" are possible:
-- - "split-usr" — /usr is a separate file system and was not mounted when elogind
--   was booted
-- - "cgroups-missing" — the kernel was compiled without cgroup support or access
--   to expected interface files is resticted
-- - "var-run-bad" — /var/run is not a symlink to /run
-- - "overflowuid-not-65534" — the kernel user ID used for "unknown" users (with
--   NFS or user namespaces) is not 65534
-- - "overflowgid-not-65534" — the kernel group ID used for "unknown" users (with
--   NFS or user namespaces) is not 65534
-- Current system is tagged as overflowgid-not-65534.