Determine how to handle polyglot files

[nightlark]

Some files can be valid when interpreted as completely different file types. As an example, Cosmopolitan Libc creates polyglot files that are native binaries that run on basically any desktop OS (Windows+macOS+Linux+FreeBSD+NetBSD+OpenBSD).

File type ID plugins probably need to return a list of recognized file types. However, some plugins using heuristics (e.g. s-rec/hex) might need a way to run last, and be given a flag to indicate that another plugin already recognized the file as something so they can avoid running.