umd0 solution: Add "OnlyEntireISOFileSystem"

Wraps around an ISOFileSystem, redirecting all the filenames to "" to
achieve the desired effect (should fix Bleach Soul Carnival 2 without resorting
to CPkmn's hack from #6638)

Probably breaks savestates as-is...
Rename to ISOBlockSystem, savestate workaround (?)
Merge pull request #6643 from hrydgard/umd0-openiso

umd0 solution: Add "OnlyEntireISOFileSystem" to fix #6638