log: Don't prefix message with timestamp on --debug if it's a continuation

If we prefix the second part of messages printed through
logmsg_perror() by the timestamp, on debug, we'll have two timestamps
and a weird separator in the result, such as this beauty:

  0.0013: Failed to clone process with detached namespaces0.0013: : Operation not permitted

Add a parameter to logmsg() and vlogmsg() which indicates a message
continuation. If that's set, don't print the timestamp in vlogmsg().

Link: https://github.com/moby/moby/issues/48257#issuecomment-2282875092
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

flow.c

@@ -279,7 +279,8 @@ void flow_log_(const struct flow_common *f, int pri, const char *fmt, ...)
 	else
 		type_or_state = FLOW_TYPE(f);
 
-	logmsg(true, pri, "Flow %u (%s): %s", flow_idx(f), type_or_state, msg);
+	logmsg(true, false, pri,
+	       "Flow %u (%s): %s", flow_idx(f), type_or_state, msg);
 }
 
 /**