Added CreateDirectory function to service/fs.cpp, and in Archive.

What I used to test: https://github.com/archshift/ctru-test
`-1` is a made up/invalid error code. However, it's the only error code used through the entire codebase, so I think it's fine to leave it as is for this PR. In #147 I'm sweeping through all of these and changing them to more appropriate codes, so I'll deal with it there.