make_ns: don't panic when adding an existing name to a namespace

When adding names to a namespace, make_ns asserts that a name is not already in the namespace. I made an error in contain when I added event: to the list of schemes in the contained namespace. Since event: is included in the namespace automatically, this caused a collision which led to a kernel assertion failure. This fix will simply return EEXIST if you add a scheme name that is in the default list, or duplicate a name in your own list.