Re-organized status messages to be more coherent and centralized.

- Reorganized all of the status message feedback from both the g-code
parser and settings modules to be centralized into two message modules:
status feedback from executing a line and warnings for misc feedback.

- Pulled out the printPgmString() messages in settings.c and placed it
into the new module. (settings_dump() not moved).

- Some other minor edits. Renaming defines, comment updates, etc.

serial.c

@@ -169,6 +169,7 @@ ISR(USART_RX_vect)
       sys.execute |= EXEC_ALARM; // Set alarm to allow subsystem disable for certain settings.
       
       // TODO: When Grbl system status is installed, set position lost state if the cycle is active.
+      // if (sys.cycle_start) { POSITION LOST } 
       
       // Immediately force stepper and spindle subsystem idle at an interrupt level.
       if (!(sys.execute & EXEC_RESET)) { // Force stop only first time.