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it's just crappy/unsafe/outdated, and should not be used.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

@@ -0,0 +1,40 @@
+{- System.Process enhancements
+ -
+ - Copyright 2012 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Utility.Process where
+
+import System.Process
+import System.Exit
+import System.IO
+
+import Utility.Misc
+
+{- Waits for a ProcessHandle, and throws an exception if the process
+ - did not exit successfully. -}
+forceSuccessProcess :: ProcessHandle -> String -> [String] -> IO ()
+forceSuccessProcess pid cmd args = do
+	code <- waitForProcess pid
+	case code of
+		ExitSuccess -> return ()
+		ExitFailure n -> error $
+			cmd ++ " " ++ show args ++ " exited " ++ show n
+
+{- Like readProcess, but allows specifying the environment, and does
+ - not mess with stdin. -}
+readProcessEnv :: FilePath -> [String] -> Maybe [(String, String)] -> IO String
+readProcessEnv cmd args environ = do
+	(_, Just h, _, pid)
+		<- createProcess (proc cmd args)
+			{ std_in  = Inherit
+			, std_out = CreatePipe
+			, std_err = Inherit
+			, env = environ
+			}
+	output  <- hGetContentsStrict h
+	hClose h
+	forceSuccessProcess pid cmd args
+	return output