syslog: Avoid flushing syslog_stream buffer, if possible, until lib_vsprintf()...

syslog: Avoid flushing syslog_stream buffer, if possible, until lib_vsprintf() completely parses the format.  This assures that the flush will flush the entire output, even data that may potentially follow the linefeed.  And, in that case, it cannot be interleaved with other devug output.  Suggested by Jussi Kivilinna.