journal: handle multiline syslog messages

Since the journal can handle multiple lines just well natively,
and rsyslog can be configured to handle them as well, there is no need
to truncate messages from syslog() after the first newline.

Reproducer:

1. Add following four lines to /etc/rsyslog.conf

   ----------
   $EscapeControlCharactersOnReceive off
   $ActionFileDefaultTemplate RSYSLOG_SysklogdFileFormat
   $SpaceLFOnReceive on
   $DropTrailingLFOnReception off
   ----------

3. Restart rsyslog
  # service rsyslog restart

4. Compile and run the following program

   ----------
   #include <stdio.h>
   #include <syslog.h>

   int main()
   {
    syslog(LOG_INFO, "aaa%caaa", '\n');
    return 0;
   }
   ----------

Actual results:
Below message appears in /var/log/messages.

   ----------
　　　Sep  7 19:19:39 localhost test2: aaa
   ----------

Expected results:
Below message, which worked prior to systemd-journald
appears in /var/log/messages.

   ----------
　　　Sep  7 19:19:39 localhost test2: aaa aaa

https://bugzilla.redhat.com/show_bug.cgi?id=855313