S3FS

@sherakama likewise with #38, this should not affect existing installs; this just prepares us to be able to use S3FS. Depending on order of operations, this may or may not merge cleanly with the simplesamlphp branch.

Please do not delete this branch after merging.

(@mistermarco FYI)
I disagree on this, although not particularly strongly. We are adding functionality, and not removing any functionality. These code updates are backwards-compatible with earlier installs, and should not break anything existing.