Allow logging to journald.

Keeps logging to stdout, but also to journald if it's available.

tracing-journal maps the target to the non-standard journald field TARGET so the module path will not be shown in default journalctl logs. They can be filtered using something like journalctl TARGET=tor_dirmgr but the lines won't actually have that module path like stdout logs do.

Maybe a [logging] section is needed in config, especially if other options are going to be added.