Tweaks and bug fixes. Increase to 3 startup blocks. Remove purge/added unlock command

- Increased the number of startup blocks to 3 for no good reason other
than it doesn't increase the flash size.

- Removed the purge buffer command and replaced with an disable homing
lock command.

- Homing now blocks all g-code commands (not system commands) until the
homing cycle has been performed or the disable homing lock is sent.
Homing is required upon startup or if Grbl loses it position. This is
for safety reasons.

- Cleaned up some of the Grbl states and re-organized it to be little
more cohesive.

- Cleaned up the feedback and status messages to not use so much flash
space, as it's a premium now.

 - Check g-code and dry run switches how are mutually exclusive and
can't be enabled when the other is. And automatically resets Grbl when
disabled.

- Some bug fixes and other minor tweaks.

report.h

@@ -34,17 +34,16 @@
 #define STATUS_SETTING_VALUE_NEG 9
 #define STATUS_SETTING_STEP_PULSE_MIN 10
 #define STATUS_SETTING_READ_FAIL 11
-#define STATUS_HOMING_ERROR 12
+#define STATUS_IDLE_ERROR 12
 #define STATUS_ABORT_CYCLE 13
-#define STATUS_PURGE_CYCLE 14
+#define STATUS_HOMING_LOCK 14
 
 // Define Grbl feedback message codes. Less than zero to distinguish message from error.
-#define MESSAGE_SYSTEM_ALARM -1
-#define MESSAGE_POSITION_LOST -2
-#define MESSAGE_HOMING_ENABLE -3
-#define MESSAGE_SWITCH_ON -4
-#define MESSAGE_SWITCH_OFF -5
-#define MESSAGE_PURGE_AXES_LOCK -6
+#define MESSAGE_CRITICAL_EVENT -1
+#define MESSAGE_HOMING_ALARM -2
+#define MESSAGE_ENABLED -3
+#define MESSAGE_DISABLED -4
+#define MESSAGE_HOMING_UNLOCK -5
 
 // Prints system status messages.
 void report_status_message(uint8_t status_code);