mu: improve command-line handling

- Remove some dead code
- Make error messages a bit more user-friendly.
- Small cleanups
- Add unit tests and actually run them

mu/mu.cc

@@ -29,8 +29,6 @@
 #include "utils/mu-utils.hh"
 #include "utils/mu-logger.hh"
 
-#include "mu-cmd.hh"
-
 using namespace Mu;