Add checks for license group mismatches

[mgorny]

I think we could add a few checks for ensuring that license_groups don't get accidentally mixed up. In particular:

• license in @MISC-FREE must not be in union of @FSF-APPROVED @OSI-APPROVED
• @MISC-FREE-DOCS not in @FSF-APPROVED-OTHER
• @BINARY-REDISTRIBUTABLE minus literal @FREE is not in expanded @FREE (i.e. we're not duplicating stuff) — maybe we should split @BINARY-REDISTRIBUTABLE into two groups to make this cleaner
• @EULA is neither in @FREE nor @BINARY-REDISTRIBUTABLE

CC @ulm

[ulm]

Sure, you can add such tests. OTOH, the file is simple enough that so far we haven't had any problems maintaining it manually.