Document the encoding of paths in filesystem functions

Currently, the documentation doesn't state anything about the contents of the const char * parameters. This can lead to confusion and two possible interpretations:

1. It's the filename exactly as stored in the filesystem. True on Linux and OS X, false on Windows.
2. It's UTF-8. True on Windows, partially true on OS X, true under normal circumstances on Linux (but could be false if a file is intentionally created with a non-UTF-8 name).

Since the purpose of libuv is to allow writing portable cross-platform code, it should discourage writing non-portable code, which is what the first interpretation could lead to. Therefore, I propose that libuv document the following:

• The filesystem is assumed to be UTF-8 on Linux and OS X and UTF-16 on Windows.
• Filenames passed and received are encoded in UTF-8, unless the above precondition is violated, in which case the encoding is not guaranteed.

See nodejs/node#5616 for an example of applying the first interpretation and potentially adding more ways of writing non-portable code to Node.js core.

Start with the linked libuv filesystem documentation and locate the descriptions of functions that accept or return const char * path parameters. Update the documentation to state the platform assumptions and UTF-8 encoding behavior proposed in the issue, including the caveat for non-UTF-8 filesystems.