metadata: recover politely if xattr/acl support is missing.

metadata: recover politely if xattr/acl support is missing.

...previously we'd just crash, which is definitely not polite.

metadata.py now prints warning on import if these features are missing.
That's probably overly obnoxious, especially on systems that don't support
those types of metadata at all.  Is there a way to determine whether a
kernel *should* support that type of metadata, so we can warn only if so?

(Obviously if the kernel doesn't support, say, xattrs, there's no point
warning that bup doesn't support them, because no files will be using them
anyway.  Hmm...)

Signed-off-by: Avery Pennarun <apenwarr@gmail.com>