journalctl: use _cleanup_ in one function
systemd-python: add _Reader.closed attribute

This should make the file interface of _Reader complete.
man/catalog: fix synopsis and remind to free
systemd-python: add _Reader.get_catalog()

This one wraps sd_journaal_get_catalog.
systemd-python: add journal.get_catalog()

This one wraps sd_journal_get_catalog_from_message_id.
Thanks to Python namespacing, we can stick to a shorter name.
systemd-python: small cleanups

- separate methods with two empty lines for clarity
- avoid malloc(0) by specyfing private data size as -1
- add method name in error messages
journalct: beef up entry listing

The ability to dump catalog entries in full and by id is added.
catalog: remove broken links to wiki

https://bugs.freedesktop.org/show_bug.cgi?id=58359