dda.c: endstop stop more reliably.
gcode_parse.h: raise S word size.

Some M-codes apparently need this. Perhaps it could be avoided,
but it costs just 8 bytes binary size and 2 bytes RAM, so not
really a reason to make a headache.
Get rid of E_STARTSTOP_STEPS.

This meant to be a firmware-provided retract feature but was
really supported by G-code generators. Without their support
(by issueing M101/M103), it's pretty hard to detect extrusion
pauses, so this feature simply has not future.

As this was on by default, it saves over 200 bytes binary size
in a default configuration.