Add DOS snake game (Allegro/DOS4G) and fix it to run in DOSBox 0.74

- mk_sfx: the tone fills write n ints into a buffer sized n bytes, a 4x
  overflow that trashed the DPMI heap and hung the next malloc (crash on
  the 2nd sample). Use a zeroed int accumulator, then convert to 8-bit
  afterwards; the fx_* generators now take int*.
- Timing: install_int(tick, BPS_TO_TIMER(100)) double-converted the rate
  (install_int expects milliseconds), so tick fired ~once per 12s and g_ms
  never advanced, freezing the snake while the keyboard still worked.
  Use install_int_ex(tick, BPS_TO_TIMER(1000)) so g_ms is in ms, matching
  the game's step_ms/state_t/life constants.
