The extrusion speed was wrong due to a not high enough precision of
esteps to XY steps, therefore now the target float values are used to
calculate the ratio between XY movement and extrusion speed.
The e_speed_multiplier8 was replaced by an absolute multiplier called
abs_adv_steps_multiplier8, therefore one multiplication and bitshift can
be saved inside the stepper ISR. Due to this, also extruder_advance_k is
better suited inside the planner and not the stepper files any more.

I stumbled across a name change that'll be used a lot so I'm updateing
the name list.

#3631 broke z dual endstops. Solved by @Nocturnal42 in https://github.com/MarlinFirmware/Marlin/issues/5078#issuecomment-256572248

Followup to #5090

・Add header
・Adjust spacing

.) Use already existing inverse_millimeters instead of /
block->millimeters.
.) Prevent overflow during acceleration calculation by checking if float
is necessary. Idea modified from Sailfish.
.) Save two uint32_t or even float multiplications by checking if
step[AXIS] has steps and if max acceleration is lower than accel. If
not, there is no need to check this axis.