homing: report how far off a motor previously was after a new home

It would be useful to know how far off a motor had been after it is rehomed.

For a galil axes, this should be straightforward. A motor is assigned a position of 0 after a home, prior to this if it is not at 0 this indicates its current misalignment from the previous home position.

## Acceptance criteria
- A "goto to home" (but not re-apply home position) option is added
- The galil driver also records the previous home misalignment before setting position to zero