Add `-n auto-1` option

[albertas]

It would be very convenient to have an option to specify that all cores except 1 have to be picked for execution.
This option could be called auto-1.

[RonnyPfannschmidt]

When implementing this we might want to consider supporting formulas to some extend for clamping, add/subtract and divide/ multiply

The details ought to be outsourced to execnet