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.

config.h

@@ -164,7 +164,7 @@
 // and addresses are defined in settings.h. With the current settings, up to 5 startup blocks may
 // be stored and executed in order. These startup blocks would typically be used to set the g-code
 // parser state depending on user preferences.
-#define N_STARTUP_LINE 1 // Integer (1-5)
+#define N_STARTUP_LINE 3 // Integer (1-5)
 
 // ---------------------------------------------------------------------------------------
 // FOR ADVANCED USERS ONLY: