Low-hanging DMD diff reduction fruit.

The trailing whitespace isn't pretty, but removes the
files completly from the diff.
More trivial diff reduction now that #484 is in.
Merge pull request #486 from klickverbot/dmd-diff

Low-hanging DMD diff reduction fruit
Renamed driver/target to .../targetmachine to avoid confusion with the frontend file.
Cleanup: We don't support DMC.
Cleanup: Kill backend_init/term.
Factored predefined version handling code out of main().
Also factored out argument parsing to separate function.

The code is still messy, but we have to start somewhere...
De-ancient-C-ification, spelled out EXIT_SUCCESS.
Removed a few fatal() calls in argument parsing code.

global.errors is checked immediately after return anyway.