Alarm if limits engaged on homing
No false alarm if other bits in port are set.
Minor settings number overflow bug fix.

- The `x` in `$x=val` would overflow when a value larger than 255 was
entered and passed to Grbl. This resulted with unintended parameters
being set by the overflow value. To fix, simply check for values larger
than 255 and error out.
Merge pull request #493 from alpharesearch/edge

If variable spindle is defined print S value via $G command.
Merge pull request #494 from ashelly/homing-alarm

Alarm if limits engaged on homing start.
Edit hard limit check at start of homing cycle
Merge branch 'edge' into laser