Adding support for M64/M65 commands
Backing out serial echo changes
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.
If variable spindle is defined print S value via $G command.
Alarm if limits engaged on homing
No false alarm if other bits in port are set.
Edit hard limit check at start of homing cycle
add MOTION_MODE_PROBE_NO_ERROR
install G38.{3,4,5}
utilize MOTION_MODE_PROBE_NO_ERROR
cleanup global var and push probe mode into probe_get_state
test only for & PROBE_ACTIVE
bump mantissa to uint16_t to enable G38.5
add/install probe_errors_enabled in mc_probe_cycle
hop over probe pull-off sequence after probe miss

and while "no errors" is enabled (G38.3, G38.5)
add probe_finalize to keep things DRY

this allows the PRB report to be valid when in "no errors" mode and the probe fails
report probe_succeeded with probe status