Add FreeBSD support.

This pull request adds very basic support for FreeBSD.  It works with the USER namespace.  Adding SYSTEM namespace support is left as further exercise by the reader.  (It isn't included in this revision primarily because it would complicate the node-xattr API, since it looks like FreeBSD may be the only one to support multiple namespaces.)

Included as an optional item is commit justcfx2u/node-xattr@da2415f, which returns a JavaScript object with the attribute name and value of null if the requested attribute is not found on the file.  This seems preferable to throwing an exception.