Auto merge of #1345 - badboy:pass-cargo-name, r=alexcrichton

Auto merge of #1345 - badboy:pass-cargo-name, r=alexcrichton

This reverts a change from ee5e24ff8b5.

Before it passed "foo" as `argv[0]`, which does not help much (consider the case there is one script symlinked to different names to handle different things).

This patch changes that to `cargo-{command}`.

It also reverts the passing of "foo" on the help command. Before it used `os::arg`, a different (and ugly) solution would be:

    let args = &[env::args().next().unwrap().clone(), "-h".to_string()];