shared: leave output_journal() output in buffer (#6304)

shared: leave output_journal() output in buffer (#6304)

e268b81e moved an fflush() from output_json() to the generic
output_journal(), when it probably should have deleted all fflush()
calls from logs-show.c altogether.

The caller supplies the FILE * to these functions, and should be in
charge of flushing as needed.  The current implementation essentially
defeats any buffering stdio was bringing to the table, resulting in
extraneous tiny write() calls in commands like `journalctl -b`.

This commit removes the fflush() call from output_journal(), and adds
them to journalctl before waiting for more entries and at completion.
This way in the hot path when journalctl loops on entries stdio can
combine multiple entries into bulkier write() calls.