SteppingChecks

Safety checks for the stepping:

• Don't slide on repeated zeroseeks (this moves all coordinates by -2 steps!)
• Report STEPPER ZEROSEEK MOVED correctly, even if starting point isn't pos=0
• In zeroseek, confirm hits before enabling absolute mode
• In zeroseek, retry zeroseek movement if cannot confirm limit switch (Fixes #11 (closed) )
• Add safety stepper_maxsteps_move in case of limit switch failure (Fixes #3 (closed) )

To be merged to GUI after !13 (merged)