Move some code to rustpkg::parse_args.rs

Rearrange some code in rustpkg:

* Move command line argument parsing to  parse_args.rs
* Introduce an enum to descibe commands such as Build, Install, and Clean.
* Move sysroot from Context to BuildContext, to make parse_args  more modular.

This is my first pull request, so please tell me if there is anything I need to do.