Minor bug fixes.

- Bug fix for step and direction invert masks not immediately being in
effect. Now regenerates the masks when a user changes this setting.

- Bug fix for probing cycle. G-code standard mandates that there is an
error if the probe is already triggered when the cycle is commanded.
However, Grbl may have motions to pull off a previous probing cycle in
queue and can falsely lead to errors. To fix this, the triggered check
is performed within the probing cycle itself, right after the planner
buffer is synced. If there is an error, it will now alarm out as a
probe fail.