logging: add support for writing to journald

[jabedude]

@lucab could you please review my initial pass at adding journald logging support.

1. I only have the core logic of forwarding messages to journald in place

2. I am not able to pass journald a memfd and create a log entry. I've done a bit of research and only found some issues in nix with passing multiple fds, but that shouldn't matter in our case.

Close: #32

[jabedude]

Got the memfd message working. It needed to have been sealed before being passed to journald.

[lucab]

@jabedude nice! I did a first review pass and run the tests, I got the correct output in the journal. I think sealing is not strictly required but it plays better with the receiving logic in systemd-journald, so I'm ok with doing that.

[jabedude]

@lucab thanks! I integrated your comments and made some small tweaks. I think the PR is ready for review!

[lucab]

I've left a few minor comments, but I plan to have a deeper pass tomorrow. Can you please squash into a single commit in the meanwhile?

[jabedude]

Sure thing

[lucab]

@jabedude thanks for all your work on this! This PR looks fine to me now. Is there anything else you planned to add here, or can I go ahead and merge?

[jabedude]

My pleasure! No, I think this is good to merge as is. I think we might want to add methods for consumers to log with arbitrary fields and data later though, but I need to look closer at sd_journal_sendv to decide on the API.