Prevent mapReduce from outputting to system collections

In some manual tests it seemed like it was not possible to output to "system.views" since we prevent renaming to/from that collection. However I was able to output to "system.js" and there's no enforcement that the schema matches (see also SERVER-39122).

This doesn't really sound like a "feature" and I think should be disallowed. $out does not allow outputting to system collections.