Fixed thin-layer Z-axis printing problem

Fixed problem whereby when printing in thin layers, the z-axis stepper
would try to move too fast, resulting in missed steps. This was because
the lookahead code was setting incorrect z-move velocities when a Z-move
was preceded by an extruder-only move.
Improve performance of SD card upload via USB

This change approximately doubles the speed of SD-card uploading via USB
to 2.2Mbyte/sec on the test system, provided that Pronterface has been
patched to not wait for an acknowledgement after sending each line of
g-code.