lib/sysroot: Add journal-msg signal

lib/sysroot: Add journal-msg signal

This will allow us to drop the awful hack in rpm-ostree where we watch our own
stdout. In general, libraries shouldn't write to stdout.

Also we can kill the systemd journal wrapper code. There's some duplication at
each call site now...but it's easier than trying to write a `sd_journal_send()`
wrapper.

I was originally going to have this emit all of the structured data too as a
`GVariant` but decided it wasn't worth it right now.

Closes: #1052
Approved by: jlebon