Flag to select -xy to mean -x -y or --x=y (-x y) if x is defined to take params

In case of an "aggregate", `-xy z` should still mean `-x z -y`, but it starts to get complicated, e.g. with multiple non-nullary options in the set. So, not a burning priority; spaces have always been there to help... Also, the naturally ordered `-yx z` already works (as `-x z -y`).