prim_file: Refactor handling of responses

Currently, the format of the return value from drv_command/3 is
determined solely by the efile driver's response. In a future
commit, we will need to produce different return values that
also dependend on which function in prim_file that was called;
thus, we will need some way to pass down some sort of state
to drv_get_response/2.

As a preparation for that, allow the third argument of drv_command/3
to be a fun. That also allows us to remove the convoluted special
case handling of the list_dir operation.

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