debug: Implement passing of command-line arguments to the debugged program.

Changes the name of client.Run to client.New to better describe what it
does; previously users had to call two functions named Run to load and
start a program.

Removes the unimplemented function SetArguments, and instead arguments
are specified in the call to Run.

Change-Id: I12c3178b431efc01841242a88feee2f69b44d330
Reviewed-on: https://go-review.googlesource.com/10564
Reviewed-by: Rob Pike <r@golang.org>