dda: Convert TARGET axis vars to array.

In preparation for more efficient and scalable code using axis-loops
for common operations, add two new array-types for signed and unsigned
32-bit values per axis. Make the TARGET type use this array instead of
its current X, Y, Z, and E variables.

Traumflug notes:

- Did the usual conversion to spaces for changed lines.

- Added X = 0 to the enum. Just for peace of mind.

- Excellent patch!

Initially I wanted to make the new array an anonymous union with the
old variables to allow accessing values both ways. This way it would
have been possible to do the transition in smaller pieces. But as
the patch worked so flawlessly and binary size is precisely the
same, I abandoned this idea. Maybe it's a good idea in other areas.
dda: Convert more axis variables to arrays.

Many places in the code use individual variables for int/uint values
for X, Y, Z, and E.  A tip from a comment suggests making these into
arrays for scalability in the future. Replace the discrete variables
with arrays so the code can be simplified in the future.
um_to_steps_*: convert to generic implementation

A generic implementation here will allow callers to pass the target axis
in as a parameter so the callers can also be made more generic.
dda: Move axis calculations into loops.

Clean up code to reduce duplication by consolidating code into
loops for per-axis actions.
home: Reduce duplication with axis arrays
Remove obsolete um_to_steps_[xyze]
datalog: show traced data as signed ints, not unsigned
Rename confusing variable name

'all_time' sounds like forever to me, but this variable really
tracks the last time we hit one of "all the axes".  It sticks
out more now in looping, so rename it to make sense.
Trivial cleanups

Move some code around, fix some formatting, and hide a couple
of variables when they're not being used.