librust: make `rust run file.rs` past any extra arguments to the compiled program.
auto merge of #5863 : huonw/rust/rust-run-args, r=thestinger

e.g. 
```
$ cat echo.rs
fn main() {
   io::println(fmt!("%?", os::args()));
}
$ rust run echo.rs 1 2 3
~[~"./echo~", ~"1", ~"2", ~"3"]
```