MEDIUM: logs: remove the hostname, tag and pid part from the logheader

At the moment we have to call snprintf() for every log line just to
rebuild a constant. Thanks to sendmsg(), we send the message in 3 parts:
time-based header, proxy-specific hostname+log-tag+pid, session-specific
message.
diff --git a/include/types/proxy.h b/include/types/proxy.h
index 170e6f7..1400126 100644
--- a/include/types/proxy.h
+++ b/include/types/proxy.h
@@ -346,6 +346,7 @@
 	struct list logsrvs;
 	struct list logformat; 			/* log_format linked list */
 	char *log_tag;                          /* override default syslog tag */
+	struct chunk log_htp;			/* a syslog header part that contains hostname, log_tag and pid */
 	char *header_unique_id; 		/* unique-id header */
 	struct list format_unique_id;		/* unique-id format */
 	int to_log;				/* things to be logged (LW_*) */