switch from System.Cmd.Utils to System.Process

Test suite now passes with -threaded!

I traced back all the hangs with -threaded to System.Cmd.Utils. It seems
it's just crappy/unsafe/outdated, and should not be used. System.Process
seems to be the cool new thing, so converted all the code to use it
instead.

In the process, --debug stopped printing commands it runs. I may try to
bring that back later.

Note that even SafeSystem was switched to use System.Process. Since that
was a modified version of code from System.Cmd.Utils, it needed to be
converted too. I also got rid of nearly all calls to forkProcess,
and all calls to executeFile, which I'm also doubtful about working
well with -threaded.

Utility/TempFile.hs

@@ -9,7 +9,7 @@ module Utility.TempFile where
 
 import Control.Exception (bracket)
 import System.IO
-import System.Posix.Process hiding (executeFile)
+import System.Posix.Process
 import System.Directory
 
 import Utility.Exception