journald: don't flush to /var/log/journal before we get asked to

This changes journald to not write to /var/log/journal until it received
SIGUSR1 for the first time, thus having been requested to flush the runtime
journal to disk.

This makes the journal work nicer with systems which have the root file system
writable early, but still need to rearrange /var before journald should start
writing and creating files to it, for example because ACLs need to be applied
first, or because /var is to be mounted from another file system, NFS or tmpfs
(as is the case for systemd.volatile=state).

Before this change we required setupts with /var split out to mount the root
disk read-only early on, and ship an /etc/fstab that remounted it writable only
after having placed /var at the right place. But even that was racy for various
preparations as journald might end up accessing the file system before it was
entirely set up, as soon as it was writable.

With this change we make scheduling when to start writing to /var/log/journal
explicit. This means persistent mode now requires
systemd-journal-flush.service in the mix to work, as otherwise journald would
never write to the directory.

See: #1397

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