Change the way the symmetric retraction defaults work for M601

After some thought and experimentation, the new default value method
make it easier to disable the pre-move retraction without also setting
the post-move extrusion to 0 in M601 (the analog of being able to set L0
in M600 without losing the default E value.. This is useful (and
provides better results) if you are only doing short parks of the head.