xhc-hb04: synthesize several per-axis buttons

Users often want to use some buttons on a per-axis basis:

So, if any of the buttons named:
   button-zero, button-goto-zero, button-half
exist, synthesize additional buttons conditioned by the rotary
switch axis selector with names:
   button-zero-x
   button-zero-y
   button-zero-z
   button-zero-a
   button-goto-zero-x
   button-goto-zero-y
   button-goto-zero-z
   button-goto-zero-a
   button-half-x
   button-half-y
   button-half-z
   button-half-a
and create corresponding hal pins:
   xhc-hb04.button-zero-x
   xhc-hb04.button-zero-y
   ...
   etc.
New message and stat entry for rapid override
HALUI: support setting rapid override in the various ways
Honor rapid override
Fix halui's rapid override increase and decrease pins
As Michael G says, this config option is unneeded.
Add rapid override support to AXIS
Fix axis.ini to be consistent about maxvel
Support rapid override "direct" input in halui, for symmetry
halui: add Rapid Override pins to manpage
Merge branch '2.6'
axis: separate buttons for system, tool touchoff
Remove the error-prone way of doing tool touch off

This removes the ability to do it on the old menu.  It led to error and
so was probably a mistake all along, so let's go ahead and be bold and
remove it.
Add keyboard shortcut, tooltip, and quickref documentation for tool touchoff