DDA: finally, don't bit-shift dda->c.

This was the goal: to not bit-shift when calling setTimer(). Binary
size another 40 bytes off, about 1.2 % better performance:

    SIZES             ATmega...  '168    '328(P)    '644(P)    '1280
    FLASH : 20136 bytes          141%       66%        32%       16%
    RAM   :  2318 bytes          227%      114%        57%       29%
    EEPROM:    32 bytes            4%        2%         2%        1%

short-moves.gcode statistics:
LED on occurences: 888.
LED on time minimum: 302 clock cycles.
LED on time maximum: 718 clock cycles.
LED on time average: 311.258 clock cycles.

smooth-curves.gcode statistics:
LED on occurences: 9124.
LED on time minimum: 307 clock cycles.
LED on time maximum: 708 clock cycles.
LED on time average: 357.417 clock cycles.

triangle-odd.gcode statistics:
LED on occurences: 1636.
LED on time minimum: 302 clock cycles.
LED on time maximum: 708 clock cycles.
LED on time average: 330.322 clock cycles.
arduino_xxx.h: add DIO names next to AIO names.

This is mostly for less confusion, because analog pins can
be used as digital ones, too. It also matches what other firmwares
do, so people can simply copy & paste pin definitions.

Definitions were taken from Sprinter's fastio.h (which was
initially crafted by copying Teacup's arduino*.h files :-) )
gcode_process.c: review G28 comment.
DDA: replace ACCELERATE_RAMP_LEN with a proper function.

This was (hopefully) the last place where this STEPS_PER_M_X-
assumption was in use.
dda_lookahead.c: get rid of a number of unused functions.

... including the last function which did the STEPS_PER_M_X
assumption.
dda_lookahead.c: note TODO on looping needs.
dda_lookahead.c: do statistics when debugging, only.

This saves a surprising 140 bytes flash and 4 bytes RAM, so
likely a nice little speed enhancement.
dda_lookahead.c: sort more statistics stuff.

Another 40 bytes flash and 8 bytes RAM off.