Simple freebsd extattr support

I've added preprocessor macros to make sure that the function calls that deal with extended attributes doesn't fail. FreeBSD extended attributes are namespaced (see [extattr(9)](http://www.freebsd.org/cgi/man.cgi?query=extattr&apropos=0&sektion=9&manpath=FreeBSD+9.0-RELEASE&arch=default&format=html)), requiring some nontrivial rewriting to have be correctly handled by metastore, so this solution is very rudimentary. You have support for pulling ext attributes from a different namespace by supplying a compile time option, see README.freebsd.