Normalise API for enabling interrupts on GPIO & MCP23017 pins

[npryce]

GPIO and MCP23017 have different APIs for configuring interrupt mode. The APIs should be the same, so pick one and make the other one use it.

[npryce]

The interrupt behaviours are different enough that the API cannot be completely compatible. For example, the MCP23017 does not raise interrupts on rising or falling signals but holds the interrupt line high while the input is different from some preconfigured value.