Setting/removing extended attributes on directories was partially fixed with
commit eff35e60b6. However, on most file systems
it is also possible to do these operations without read access (see tests).
Since we cannot open a write-access fd to a directory, we have to use the
/proc/self/fd trick (already used for ListXAttr) for the other operations aswell.
For simplicity, let's separate the Linux and Darwin code again (basically revert
commit f320b76fd1), and always use the
/proc/self/fd trick on Linux. On Darwin we use the best-effort approach with
openBackingFile() as a fallback.
More discussion about the available options is available in
https://github.com/rfjakob/gocryptfs/issues/308.
An Open() a fifo blocks until it is opened for writing.
This meant that xattr operations on FIFOs would block.
Pass O_NONBLOCK to fix that, and add a test.
Values a binary-safe, there is no need to base64-encode them.
Old, base64-encoded values are supported transparently
on reading. Writing xattr values now always writes them binary.
