Change "Software" to "System" in SPDX

[zvr]

This changes SPDX to expand to "System" instead of "Software".

In:

• top-level README (title and first paragraph description)
• spec title (in docs/index.md) -- also removes "DRAFT"
• scope (in docs/scope.md)
• top-level setup.py

[kestewart]

Thanks Alexios.

Changes look good, and much appreciate you getting them drafted up and included.