Add a mach update command that invokes cargo update for all ports (fixes #3736)

Example usage:

``` sh
$ ./mach update-cargo -p rust-xml
.
    Updating git repository `https://github.com/netvl/rust-xml`
ports/cef
    Updating git repository `https://github.com/netvl/rust-xml`
ports/android/glut_app
    Updating git repository `https://github.com/netvl/rust-xml`
```