rgw_file: expose RGW user-defined attributes

rgw_file:  expose RGW user-defined attributes

Permit nfsv4.2-style operations on RGW's user-defined attributes
(only).

RGW requires that user-defined attribute keys are prefixed
with RGW_ATTR_META_PREFIX.  Add/remove this prefix automagically
when storing/retrieving.

A special set of read-only,  internal attributes--currently just
user.rgw.etag--will be sent with lsxattrs() result.

Adds librgw_file_xattr fixture with unit tests for setxattrs,
getxattrs, lsxattrs, rmxattrs.

rgw_file: fix exposed_xattr support

Also extends the unit test to cover the missed cases.

Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>