Overhall System.Process

       
 - fix #1780: pipes created by runInteractiveProcess are set
   close-on-exec by default
 
 - add a new, more general, form of process creation: createProcess
   Each of stdin, stdout and stderr may individually be taken
   from existing Handles or attached to new pipes.  Also it
   has a nicer API (as discussed on libraries@haskell.org).
 
 - add readProcess / readProcessWithExitCode, originally from Don
   Stewart's newpopen package.  These functions behave like C's
   popen().

 - Move System.Cmd.{system,rawSystem} into System.Process.  Later
   we can depecate System.Cmd.
 
 - Don't use O_NONBLOCK for pipes, as it can confuse the process
   attached to the pipe (requires a fix to GHC.Handle in the base
   package).

 - Provide a way to close all the file descriptors in the new
   process (see #1415)

 - add a couple more tests for the new features

 - bump the version to 2.0