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/runtime/haskell-bind/PGF2.hsc

@@ -15,7 +15,7 @@
 module PGF2 (-- * CId
              CId,
              -- * PGF
-             PGF,readPGF,AbsName,abstractName,startCat,
+             PGF,readPGF,AbsName,abstractName,Cat,startCat,
              -- * Concrete syntax
              ConcName,Concr,languages,parse,parseWithHeuristics,
              hasLinearization,linearize,linearizeAll,alignWords,