gf -cshell: preliminary support for the C run-time system in the GF shell

Some C run-time functionality is now available in the GF shell, by starting
GF with 'gf -cshell' or 'gf -crun'. Only limited functionality is available
when running the shell in these modes:

- You can only import .pgf files, not source files.
- The -retain flag can not be used and the commands that require it to work
  are not available.
- Only 18 of the 40 commands available in the usual shell have been
  implemented. The 'linearize' and 'parse' commands are the only ones
  that call the C run-time system, and they support only a limited set of
  options and flags. Use the 'help' commmands for details.
- A new command 'generate_all', that calls PGF2.generateAll, has been added.
  Unfortuntaly, using it causes 'segmentation fault'.

This is implemented by adding two new modules: GF.Command.Commands2 and
GF.Interactive2. They are copied and modified versions of GF.Command.Commands
and GF.Interactive, respectively. Code for unimplemented commands and other
code that has not been adapted to the C run-time system has been left in
place, but commented out, pending further work.

src/compiler/GF/Infra/Option.hs

@@ -73,7 +73,10 @@ errors = raise . unlines
 
 -- Types
 
-data Mode = ModeVersion | ModeHelp | ModeInteractive | ModeRun | ModeCompiler
+data Mode = ModeVersion | ModeHelp
+          | ModeInteractive | ModeRun
+          | ModeInteractive2 | ModeRun2
+          | ModeCompiler
           | ModeServer {-port::-}Int
   deriving (Show,Eq,Ord)
 
@@ -302,6 +305,8 @@ optDescr =
      Option ['j'] ["jobs"] (OptArg jobs "N") "Compile N modules in parallel with -batch (default 1).",
      Option [] ["interactive"] (NoArg (mode ModeInteractive)) "Run in interactive mode (default).",
      Option [] ["run"] (NoArg (mode ModeRun)) "Run in interactive mode, showing output only (no other messages).",
+     Option [] ["cshell"] (NoArg (mode ModeInteractive2)) "Start the C run-time shell.",
+     Option [] ["crun"] (NoArg (mode ModeRun2)) "Start the C run-time shell, showing output only (no other messages).",
      Option [] ["server"] (OptArg modeServer "port") $
        "Run in HTTP server mode on given port (default "++show defaultPort++").",
      Option [] ["document-root"] (ReqArg gfDocuRoot "DIR")