journal: do server_vacuum for sigusr1

runtime journal is migrated to system journal when only
"/run/systemd/journal/flushed" exist. It's ok but according to this
the system journal directory size(max use) can be over the config. If
journal is not rotated during some time the journal directory can be
remained as over the config(or default) size. To avoid, do
server_vacuum just after the system journal migration from runtime.