Mark M600/M601 arguments more inituitive.

The original M600 command used 'L' (assumedly for "long
retract/extrude"). This commit changes the L & E to a more intuitive 'R'
for pre-move Retract and 'E' for post-move Extrude without any implied
behavior. The removal of the implied behavior (where one retract/extrude
movement value is implied from the other if not specified) was perhaps
cleverer but this is felt to be outwieghed by the fact that the new
default argument value scheme is simpler and consistent with other
arguments (KISS).