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/log.h b/include/types/log.h
index 0214ae6..2cfd31a 100644
--- a/include/types/log.h
+++ b/include/types/log.h
@@ -37,6 +37,13 @@
 /* The array containing the names of the log levels. */
 extern const char *log_levels[];
 
+/* enum for log format */
+enum {
+	LOG_FORMAT_RFC3164 = 0,
+	LOG_FORMAT_RFC5424,
+	LOG_FORMATS,          /* number of supported log formats, must always be last */
+};
+
 /* lists of fields that can be logged */
 enum {
 
@@ -158,6 +165,7 @@
 struct logsrv {
 	struct list list;
 	struct sockaddr_storage addr;
+	int format;
 	int facility;
 	int level;
 	int minlvl;