journalctl: add new --sync switch for syncing the journal to disk

journalctl: add new --sync switch for syncing the journal to disk

With this new "--sync" switch we add a synchronous way to sync
everything queued to disk, and return only after that's complete. This
command gives the guarantee that anything queued before has hit the disk
before the command returns.

While we are at it, also improve the man pages and help text for
journalctl a bit.