raspi-config: add way to disable swap

[lurch]

I notice that swap is now enabled by default whereas on previous distros it was disabled by default (yes, I've already seen that it's moved from a swap-partition to a swap-file). Maybe raspi-config should gain an option to enable/disable swap (primarily useful for people with slow SD cards, I'd imagine).

[asb]

This would be a good idea. I'll just add that although swap is enabled by default we set a very low swappiness.