Makefile-AVR: Update MCU_TARGET=at90usb1286 conditional.
gcode_process.c, heater.[hc]: Add M135 Pnn Svv [Xttt]; Set PID constants by Proportional Range and optionally, integral time.
heater.c: M135: Set I_Limit to match integral term.
heater.c: Increase PID integrator to int32_t.
heater.c: Lower DEFAULT_I to 64 for a Ki=0.0625 and a default integration time constant of 32 seconds, and max 75% I contribution.