Command is now an enum
Move syslog to BuildContext
Move command line parsing to parse_args.rs
Fix some of the testcases
simple-lib and deeply and c-dependencies still have problems.
But they were nt caused by this pull request
auto merge of #11327 : nielsle/rust/rustpkg_argparse, r=alexcrichton

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.