- `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.

Saves about 2669 bytes when deactivated. (About 1% for a AT2560, about __4%__ for a AT644!)

Add a forgotten condition

Reference: https://github.com/MarlinFirmware/Marlin/pull/3713#issuecomment-218333678