Various improvements

Tidied up axis homing tracking.
When checking endstops, wait for move to complete before accepting
further moves, otherwise subsequent moves use the wrong coordinates.
Temperatures and Z probes are now monitored continuously using a tick
interrupt to kick off ADC conversions. ADC is now run in 12-bit mode.
Thermistor readings are passed through averaging filters. Thermistors
are monitored for overheat conditions and bad readings in the tick ISR
and the appropriate heater is turned off (useful because the main loop
sometimes gets suspended while trying to do USB communication).
Use watchdog timer to monitor the tick interrupt - needs patch to
Arduino Due core library. Add facility to test watchdog timer (M111
S1001). Added an error status word to record that errors have occurred
(e.g. over-temperature).
M111 command changes so that S0 turns debug off, S1 turns debug on, S2
reports free memory - also now reports the type of the last restart and
the error status word. Fixed problem whereby M111 debug reports were not
sent to the web interface.
Implemented M999 command, which resets the Duet.
Removed an unused variable.
Changed some more "char*" to "const char*".
Changed extruder PID parameters and added more explanation for them.