Add swapoff flag

[pmaterer]

This is a fix for #450.

Adds a flag --swapoff which enables the features added in #421.

So now, by default, swapoff -a will not run during the reboot phase. Only when you have a ZFS swap device would you need to pass the --swapoff flag. This way users who are using a swap device during the install won't have things break.

[Mic92]

What I don't like about this change is that it breaks the export the when people have swap on ZFS.
People shouldn't need to have adding a flag to make nixos-anywhere work.

[Mic92]

I think something like this is more sensible:

Suggested change

	swapoff -a
	# turn all swap devices that are baked by zfs.
	swapon --show --noheadings | awk '/^\/dev\/zd/ {print $1}' | while read device; do
	swapoff "$device"
	done

You would need to have your system not have swap in zfs. However you need to do this anyway because with 1GB you would run into deadlock issues with zfs if you would try to have it in the pool.