quad_gantry_level: Limit maximum adjustment

safety feature that prevents quad_gantry_level from trying to do a
correction that might break things - if your probe fires early for
whatever reason and tries to do an 8mm correction in one corner instead
we abort if a correction is over a configurable limit by default 4mm

configurable via `max_adjust` parameter in the config

Signed-off-by: John "Fess" Fessenden <fess@fess.org>

config/example-extras.cfg

@@ -371,6 +371,9 @@
 #horizontal_move_z: 5
 #   The height (in mm) that the head should be commanded to move to
 #   just prior to starting a probe operation. The default is 5
+#max_adjust: 4
+#   Saftey limit if an ajustment greater than this value is requested
+#   quad_gantry_level will abort.
 #samples: 1
 #   Number of probe samples per point. The defaut is 1
 #samples_result: average