Support additional prefixes, like memstick:.

I also found umd01: and host01: work, etc.  May help #5377 and possibly
others.

It seems like games can assign these at will and potentially even
subpaths, but reporting hasn't shown many cases yet.  We can still watch
it.

Avoided mapping additional systems because savestates don't handle that
well currently.
Merge pull request #5393 from unknownbrackets/io-minor

Support additional prefixes, like memstick: