Respect taskset CPU limit

[user202729]

Currently, for machines with N processors, ninja by default uses N+1 processes.

However, if taskset -c 0-2 ninja is run, ninja tries to use N+1 processes anyway instead of 3+1.

(of course in this case it's possible to just pass -j3, but if it's deeply nested there isn't any way to configure it. It's not possible to control it from environment variable #1482)