Append disc label to the first disc of two-disc games too, rather than
only labelling the second disc.

Same behavior, but less code.

Previously, we had WBFS and CISO which both returned an upper bound
of the size, and other formats which returned an accurate size. But
now we also have NFS, which returns a lower bound of the size. To
allow VolumeVerifier to make better informed decisions for NFS, let's
use an enum instead of a bool for the type of data size a blob has.

Yes, that's right! It's time to add even more NKit warnings,
because users still don't understand what NKit is or how it works!

More specifically, some users seem to be under the impression that
converting an NKit file to for instance RVZ using Dolphin's convert
feature will result in a normal RVZ file, when it in fact results in
an NKit RVZ file (since NKit is not a container format in the sense
that GCZ/WIA/RVZ/WBFS/CISO is, but rather a kind of trimmed ISO).
I can hardly blame users for not knowing this, because it's not
intuitive unless you know the technical details of how NKit works.

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.

To make people more aware that they're not using a normal disc image.