Apply `ENABLED` / `DISABLED` macros to files needing only a small
number of changes.

- Init `zprobe_zoffset`
- Remove `current_position[Z_AXIS] = zprobe_zoffset` lines from the
`set_bed_level_equation_*` functions
- Apply standards to `mesh_bed_leveling` files
- Document `MESH_BED_LEVELING`