With the the current definition of echo_command I cannot compile RCBugFix (Arduino IDE 1.8.1) with the error "invalid conversion from 'const char*' to 'char*'".  This change resolves that.

All the changes to get the code into compliance with Marlin coding
standards made UBL sick.   This pretty much gets UBL back up and
running.

Made the double touch portion a conditional compile based on the
PROBE_DOUBLE_TOUCH flag.

==============================================

Bugfix

The current G38 only stopped a move if it involved the Z axis.

Moved all the G38 code to it's own section and put it where it would
always be executed no matter what axis was moving or if the endstop was
enabled.

Also added a comment to configuration_adv to alert the user the double
tap had to be turned on.

==============================================

Change G38 back to using Z_MIN_PROBE

There's no Z_MIN endstop if Z_DUAL_ENDSTOPS is enabled and you have them
set to the top of the gantry.

G38 started out as using the Z_MIN_PROBE pin.  I don't remember why we
changed it to the Z_MIN endstop.

Sometimes after killing heaters are still on.

Thanks to @ospalh -- #6056

No more mystery halts & resets

Moved messages to language.h

Added current command to inactive timeout message

The tool_change function saves the current_position to the destination
array soon after starting.  Later in the switching extruder section, the
destination array is modified when moving the Z axis up & down.  A later
section of tool_change moves the head back to the “original location”
using the destination array.  This later section assumes that the
destination array hasn’t been modified.

The fix is to save the destination Z position and then restore it after
the Z movements have completed.

Going back to using the current_position array for the switching
extruder Z axis moves (and  leaving the destination array untouched)
doesn’t fix the problem.

This bug was introduced by the “Make tool_change kinematic compatible”
commit # 847429eff4 which was merged on 10
Oct 2016 as part of PR 4982.

This bug was discovered in Issue 5966.

Pre-compute the combined position shift and home offset to save a
single float fetch-and-add per conversion. Great for delta/scara and
bed leveling.