reducing dmd help

[Jesse Phillips]

I think it would be a good idea to reduce the output from just typing dmd.
The list of switches has become become so large that it scrolls the
terminal. I think that when dmd is typed their should just be the most
important information.

* copyright
* usage
* D source file
* -c
* -D
* -d
* --help
* -release
* -v
* -v1
* -w
* -gc

--help could list out the current help. I'm not sure these are the best
choices, but I think simplifying it with the more common ones is a good
idea.