Add a logging subsystem and change all DPRINTF to ISCSI_LOG

Add a mechanism where we can set a logging subsystem that libiscsi can use.
Create an example 'log to stderr' that utilities can use for convenience.

include/iscsi-private.h

@@ -127,7 +127,9 @@ struct iscsi_context {
 	int lun;
 	int no_auto_reconnect;
 	int reconnect_deferred;
-	int debug;
+
+	int log_level;
+	iscsi_log_fn log_fn;
 };
 
 #define ISCSI_PDU_IMMEDIATE		       0x40
@@ -284,6 +286,16 @@ struct scsi_task *iscsi_scsi_get_task_from_pdu(struct iscsi_pdu *pdu);
 
 void iscsi_set_noautoreconnect(struct iscsi_context *iscsi, int state);
 
+#define ISCSI_LOG(iscsi, level, format, args...) \
+	do { \
+		if (level <= iscsi->log_level && iscsi->log_fn) { \
+			iscsi_log_message(iscsi, level, format, ## args); \
+		} \
+	} while (0)
+
+void
+iscsi_log_message(struct iscsi_context *iscsi, int level, const char *format, ...);
+
 #ifdef __cplusplus
 }
 #endif