Avoid positional arguments that are true by default

A boolean positional argument that's true by default is not nice to set to false.

For example, with the `--rel-abspath` argument in the IOL importer it's not immediately clear how to set this to a false-y value (I usually use `--rel-abspath=0`).

A better design is to use a state that's false by default, so it can simply be enabled by passing `--whatever`. The argument name can be inverted as appropriate if the default state needs to be truth-y.