MEDIUM: logs: add a new RFC5424 log-format for the structured-data

This patch adds a new RFC5424-specific log-format for the structured-data
that is automatically send by __send_log() when the sender is in RFC5424
mode.

A new statement "log-format-sd" should be used in order to set log-format
for the structured-data part in RFC5424 formatted syslog messages.
Example:

    log-format-sd [exampleSDID@1234\ bytes=\"%B\"\ status=\"%ST\"]
diff --git a/include/types/log.h b/include/types/log.h
index 2cfd31a..6a68b5d 100644
--- a/include/types/log.h
+++ b/include/types/log.h
@@ -30,7 +30,7 @@
 
 #define NB_LOG_FACILITIES       24
 #define NB_LOG_LEVELS           8
-#define NB_MSG_IOVEC_ELEMENTS   3
+#define NB_MSG_IOVEC_ELEMENTS   4
 #define SYSLOG_PORT             514
 #define UNIQUEID_LEN            128