Add tests for setxattr, getxattr, listxattr, and removexattr, and clean up the MS code for doing so.
WIP: addressing issue #87.  Extended attributes now have owners and permission bits separate from their MSEntrys.  This is to allow user A to get/set attributes on user B's objects, which is important for addressing issues such as controlling driver behavior without B's cooperation.  This is also to support more advanced AGs which need to synchronize domain-specific metadata independently of data (such as an iRODS AG).