- `per_sq_second` => `per_s2`
- `per_sqr_second` => `per_s2`
- `axis_steps_per_sqr_second` => `max_acceleration_steps_per_s2`

This is an update of MarlinDev PR #196.

G20/21: support for switching input units between millimeters and
inches.
M149: support for changing input temperature units.

In support of these changes, code_value() and code_value_short() are
replaced with an array of functions which handle converting to the
proper types and/or units.

Based on MarlinFirmware/MarlinDev#200 from @eboston

As reported by https://github.com/MarlinFirmware/Marlin/pull/3182#issuecomment-222595488

Extend M421 with I and J parameters

Remove SF_ARC_FIX from G5 (Cubic B-spline).
SF_ARC_FIX is a fix for G2/G3 (arcs) where Skeinforge always produced relative instead of absolute coordinates for the endpoint of an arc.
It's very unlikely this is also a problem with Cubic B-splines. More likely is copying from the G2/G3 code.

No compromises for the manage_heater(). manage_heater() will return immediately when there is nothing to do, but needs a constant detaT to work proper.
Calling idle() only every 200ms results in a display update every ~2 seconds - that should be enough.
For the other functionalities in idle() and manage_inactivity() 200ms is a lot but hopefully works.

Minor DELTA segmentation speedup by pulling calculations out of the loop.

Idle during long arcs
to prevent from watchdog resets during high segmented fast arcs.