journalctl: optionally delay --follow exit for a journal synchronization

Let's optionally issue a Varlink Synchronize() call in --follow mode
when asked to terminate. This is useful so that the tool can be called
and it is guaranteed it processed all messages generated before the
request to exit before it exits.

We want this in "systemd-run -v" in particular, so that we can be sure
we are not missing any log output from the invoked service before it
exits

src/shared/fork-journal.c

@@ -127,7 +127,8 @@ int journal_fork(RuntimeScope scope, const char * const *units, PidRef *ret_pidr
                         "-q",
                         "--follow",
                         "--no-pager",
-                        "--lines=1");
+                        "--lines=1",
+                        "--synchronize-on-exit=yes");
         if (!argv)
                 return log_oom_debug();