logging: move string to the beginning of msg

this allow (to some extents) to print log messages directly to
stdout and/or stderr without extra parsing.

NOTE:
- messages are sent from differnet threads and there for they are less readable
  when they arrive all together
- there is no printing of message level and subsystem

for simple coding/debugging purposes this is useful, but otherwise
look at how ping_test.c or kronosnetd create a logpipe and read
sequencial messages from there.

Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>

TODO

@@ -59,8 +59,6 @@ libknet:
  - (issue) review locking handling policies (attempt locks vs deadlocks and so on)
            almost done, needs knet_link_* bits done
  - (issue) add .3 man pages
- - (issue) fix logging packet format to allow direct output to a fd
-           (change packet structure to have string first basically)
 
 libtap:
  - (rfe)   consider adding dhcp support for tap device

libknet/libknet.h

@@ -496,9 +496,9 @@ int knet_link_get_status(knet_handle_t knet_h,
 #define KNET_MAX_LOG_MSG_SIZE    256
 
 struct knet_log_msg {
+	char	msg[KNET_MAX_LOG_MSG_SIZE - (sizeof(uint8_t)*2)];
 	uint8_t	subsystem;	/* KNET_SUB_* */
 	uint8_t msglevel;	/* KNET_LOG_* */
-	char	msg[KNET_MAX_LOG_MSG_SIZE - (sizeof(uint8_t)*2)];
 };
 
 void knet_set_log_level(knet_handle_t knet_h, uint8_t subsystem, uint8_t level);

libknet/logging.c

@@ -9,6 +9,7 @@
 #include "config.h"
 
 #include <strings.h>
+#include <string.h>
 #include <unistd.h>
 #include <pthread.h>
 #include <stdarg.h>
@@ -117,6 +118,7 @@ void log_msg(knet_handle_t knet_h, uint8_t subsystem, uint8_t msglevel,
 	    (msglevel > knet_h->log_levels[subsystem]))
 			return;
 
+	memset(&msg, 0, sizeof(struct knet_log_msg));
 	msg.subsystem = subsystem;
 	msg.msglevel = msglevel;