MINOR: log: add logger flags

Logger struct may benefit from having a "flags" struct member to set
or remove different logger states. For that, we reuse an existing
4 bytes hole in the logger struct to store a 2 bytes flags integer,
leaving the struct with a 2-bytes hole now.

include/haproxy/log-t.h

@@ -231,10 +231,16 @@ struct log_target {
 	uint16_t flags;
 };
 
+enum logger_flags {
+	LOGGER_FL_NONE     = 0x00,
+};
+
 struct logger {
 	struct list list;
 	struct log_target target;
 	struct smp_info lb;
+	uint16_t flags;
+	/* 2 bytes hole */
 	enum log_fmt format;
 	int facility;
 	int level;