Add initial support for @ subvolumes for BTRFS partitions

[ElliotLurie]

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This is a quick modification to the installer script that automatically creates @ subvolumes on BTRFS partitions. I mentioned in my issue (#205) that zstd compression could (and should) be enabled for better performance if zstd was part of base-voidstrap. So, if this change was made, this might not be the final version.

[hervyqa]

Cool..