rados: don't add \n to getxattr

Signed-off-by: Sage Weil <sage@redhat.com>
rados: optionally read setxattr value from stdin

Signed-off-by: Sage Weil <sage@redhat.com>
qa/workunits/rados/test_rados_tool: add a few xattr tests

Signed-off-by: Sage Weil <sage@redhat.com>
rados: use STD{IN,OUT}_FILENO for magic values

Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #2218 from ceph/wip-rados-xattr

rados: fix get/setxattr commands up

Reviewed-by: Yehuda Sadeh <yehuda@redhat.com>