Improvements to homing / leveling

[thinkyhead]

See individual commits for a summary of each. Basically…

Bugs:

• Fix instances where LOGICAL_Z_POSITION should be applied to a "raw" position.
• Make sure to set the software endstops when EEPROM loaded or defaults restored.
• Account for the probe Z offset when doing pre-probe move "close to the bed."

Adjustments:

• Consolidate repeated probing code into a function.
• Apply suggested fix to xSpacing / ySpacing to get better probe points.

References: #4577, #4220, #4520, #4644

[Roxy-3D]

Do we know if this fixes (or at least helps) the Z-Offset issues we have been hearing about? If so... I need to cross these changes over to the UBL code base.

[thinkyhead]

We will receive confirmation (or not) pretty soon.
Either way, the bug-fix changes here are definitely needed.