Core/Misc: Implement CMSG_SET_TAXI_BENCHMARK_MODE to toggle PLAYER_FLAGS...

..._TAXI_BENCHMARK based on arguments. To enable/disable this, call '/script SetTaxiBenchmarkMode(1)' (1 = on, 0 = off). Basically debugs the taxi flight and outputs min, max and avg FPS during flight. Automatically toggled off when flight ends.