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it's just crappy/unsafe/outdated, and should not be used.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.

Build/Configure.hs

@@ -4,7 +4,7 @@ module Build.Configure where
 
 import System.Directory
 import Data.List
-import System.Cmd.Utils
+import System.Process
 import Control.Applicative
 import System.FilePath
 
@@ -71,7 +71,7 @@ getVersionString = do
 
 getGitVersion :: Test
 getGitVersion = do
-	(_, s) <- pipeFrom "git" ["--version"]
+	s <- readProcess "git" ["--version"] ""
 	let version = unwords $ drop 2 $ words $ head $ lines s
 	return $ Config "gitversion" (StringConfig version)