Skip calling tool change code if current and selected tool number is the same.

Added 5s timeout/abort handling when waiting for index pulses prior to startiong spindle synchronized motion.
Added definitions for M401 (deploy probe) and M402 (stow probe) for plugin use.
Added support for probe protected message and alarm. Requires driver support for interrupt handled probe input.

modbus.h

@@ -34,6 +34,9 @@
 #include <stdint.h>
 #include <stdbool.h>
 
+#define MODBUS_SET_MSB16(v) ((v) >> 8)
+#define MODBUS_SET_LSB16(v) ((v) & 0xFF)
+
 typedef enum {
     Modbus_InterfaceRTU = 0,
     Modbus_InterfaceASCII,
@@ -96,6 +99,8 @@ bool modbus_enabled (void);
 void modbus_flush_queue (void);
 void modbus_set_silence (const modbus_silence_timeout_t *timeout);
 bool modbus_send (modbus_message_t *msg, const modbus_callbacks_t *callbacks, bool block);
+uint16_t modbus_read_u16 (uint8_t *p);
+void modbus_write_u16 (uint8_t *p, uint16_t value);
 bool modbus_register_api (const modbus_api_t *api);
 
 #endif