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/Command/Commands.hs

@@ -46,6 +46,7 @@ import Data.List(intersperse,nub)
 import Data.Maybe
 import qualified Data.Map as Map
 --import System.Cmd(system) -- use GF.Infra.UseIO.restricedSystem instead!
+import GF.System.Process
 import Text.PrettyPrint
 import Data.List (sort)
 --import Debug.Trace
@@ -866,12 +867,15 @@ allCommands = Map.fromList [
      synopsis = "send argument to a system command",
      syntax   = "sp -command=\"SYSTEMCOMMAND\", alt. ? SYSTEMCOMMAND",
      exec = \_ opts arg -> do
+       let syst = optComm opts  -- ++ " " ++ tmpi
+       {-
        let tmpi = "_tmpi" ---
        let tmpo = "_tmpo"
        restricted $ writeFile tmpi $ toString arg
-       let syst = optComm opts  -- ++ " " ++ tmpi
        restrictedSystem $ syst ++ " <" ++ tmpi ++ " >" ++ tmpo
        fmap fromString $ restricted $ readFile tmpo,
+       -}
+       fmap fromString . restricted . readShellProcess syst $ toString arg,
      flags = [
        ("command","the system command applied to the argument")
        ],

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