Don't warn about 'pub use' statements
Fix using the #[]-style attribute for unused imports
auto merge of #5104 : alexcrichton/rust/fix-unused-import-pub, r=graydon

The first commit fixes warnings about `pub use` imports because it can't be known whether those are actually used or not.

The second commit fixes using `#[level(unused_imports)]` style control over the emission of warnings. Before it looked like it only worked as a command-line flag.