Fix Windows path handling and simplify path normalization.

[twistor]

Issues:

1. Just because a path contains a backslash, doesn't mean it's a Windows path. Backslashes are valid in file/directory names on Linux.
2. Windows is fine using forward slashes for paths.
3. The regex was hard to figure out. This, to me at least, is a lot simpler to follow.
4. Added a test to check about removing null bytes.

Even though Linux supports backslashes, I haven't checked all the other adapters to see what they support. And, since backslashes are directory separators on Windows, we could wind up with different file structures if we allowed them. This seems like the lowest common denominator.

Also, I've merged normalizePath() and normalizeRelativePath() since I don't see a safe subset for normalizeRelativePath() to do, or how they are really different.