[plt-scheme] Trouble killing sub-processes

Fixed the problem by using process* instead of process.  process calls a 
shell which calls the desired program.

scheme->cmd->child

 Doing a kill only kills cmd, leaving the processes and ports in a weird 
state.  process* skips the command shell step, giving

scheme->child

which is what I expected in the first place.  A silly mistake, but now 
it's in the archive for the next person who does this.