GF shell: improved system_pipe (aka "?") command

1. No temporary files are created.

2. The output of a system command is read lazily, making it feasible to 
   process large or even infinite output, e.g. the following works as
   expected:

	? "yes" | ? "head -5" | ps -lextext

src/compiler/GF/System/Process.hs

@@ -0,0 +1,18 @@
+module GF.System.Process where
+import System.Process
+import System.IO(hGetContents,hClose,hPutStr)
+import Control.Concurrent(forkIO)
+import GF.System.Catch(try)
+
+-- | Feed some input to a shell process and read the output lazily
+readShellProcess :: String     -- ^ shell command
+                  -> String    -- ^ input to shell command
+                  -> IO String -- ^ output from shell command
+readShellProcess cmd input =
+  do (Just stdin,Just stdout,Nothing,ph) <-
+         createProcess (shell cmd){std_in=CreatePipe,std_out=CreatePipe}
+     forkIO $ do try $ hPutStr stdin input
+                 try $ hClose stdin
+                 waitForProcess ph
+                 return ()
+     hGetContents stdout