MEDIUM: logs: add support for RFC5424 header format per logger

The function __send_log() iterates over senders and passes the header as
the first vector to sendmsg(), thus it can send a logger-specific header
in each message.

A new logger arguments "format rfc5424" should be used in order to enable
RFC5424 header format. For example:

    log 10.2.3.4:1234 len 2048 format rfc5424 local2 info
diff --git a/include/types/proxy.h b/include/types/proxy.h
index 1400126..9793a29 100644
--- a/include/types/proxy.h
+++ b/include/types/proxy.h
@@ -346,7 +346,8 @@
 	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 */
+	struct chunk log_htp;			/* a syslog header part that contains hostname, log_tag and pid (RFC3164 format) */
+	struct chunk log_htp_rfc5424;		/* a syslog header part that contains hostname, log_tag and pid (RFC5424 format) */
 	char *header_unique_id; 		/* unique-id header */
 	struct list format_unique_id;		/* unique-id format */
 	int to_log;				/* things to be logged (LW_*) */