Support for a directory-less S3

[dstufft]

Right now if you want software to be able to handle OSFS in addition to something like S3FS you have to use things like makedir, however there are no directories on S3. The S3FS implementation works around this by creating empty files to "fake" a directory. It would be great if it were possible to just tell S3FS to no-op directories since they are unneeded.