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.

gf.cabal

@@ -132,13 +132,14 @@ Library
     PGF.OldBinary
 
   if flag(c-runtime)
-    exposed-modules:     PGF2
-    other-modules:       PGF2.FFI
-    hs-source-dirs:      src/runtime/haskell-bind
-    build-tools:         hsc2hs
-    extra-libraries:     gu pgf
-    c-sources:           src/runtime/haskell-bind/utils.c
-    cc-options:          -std=c99
+    exposed-modules: PGF2
+    other-modules:   PGF2.FFI
+                     GF.Interactive2 GF.Command.Commands2
+    hs-source-dirs:  src/runtime/haskell-bind
+    build-tools:     hsc2hs
+    extra-libraries: gu pgf
+    c-sources:       src/runtime/haskell-bind/utils.c
+    cc-options:      -std=c99
 
 ---- GF compiler as a library: