Matching changes from #8725

[1.1.x] Improved Planner::reverse_pass

Plus: 3 times 2 float / to 1 float / and 2 float *
      and, reciprocal is an optimized operation

[bugfix-1.1.x] minor planner.cpp speed improvement (follow up to COREXY stutter changes PR #8697)

Followup to #8713

[1.1.x] Fix for #8604

Fix indentation, use min for constraint, remove obsolete error check

* Better off mesh behaviour for UBL

* Spacing changes...

[1.1.x] Discard "continued" blocks on interrupted move

Based on #8685

Fix - Conditionals_post.h - allow overriding of [min,max] probe point

[1.1.x] MULTIPLE_PROBING — Why stop at 2?

[1.1.x] Purge blocks on endstop/probe hit

[1.1.x] Cleanup, bugfixes, parity with 2.0.x

[1.1.x] Don't split first_move while homing or probing

While homing or probing it might be bad if the stop/trobe triggers during the first part and the second is still in the buffer.

Addressing #8676

[1.1.x] Change K1 to required option PID_K1