journal/vacuum: cleanup
journald: fix space limits reporting

Reporting of the free space was bogus, since the remaining space
was compared with the maximum allowed, instead of the current
use being compared with the maximum allowed. Simplify and fix
by reporting limits directly at the point where they are calculated.

Also, assign a UUID to the message.
journald: always vacuum empty offline files

Corrupted empty files are relatively common. I think they are created
when a coredump for a user who never logged anything before is
attempted to be written, but the write does not succeed because the
coredump is too big, but there are probably other ways to create
those, especially if the machine crashes at the right time.
Non-corrupted empty files can also happen, e.g. if a journal file is
opened, but nothing is ever successfully written to it and it is
rotated because of MaxFileSec=. Either way, each "empty" journal file
costs around 3 MB, and there's little point in keeping them around.