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Fixes some gratuitous warnings when compiling via C with -fhpc

This lets us gain coverage on programs that use fork, like xmonad.

* To be merged to STABLE *

* To be merged to STABLE. *

foreign import ccall unsafe hs_hpc_write :: CString -> IO ()
foreign import ccall unsafe hs_hpc_read  :: CString -> IO ()

These write a Hpc description of the state of the world to a file, 
or read a description into the current Hpc tickbox subsystem.

 - .tix files are now a list of MixModule, which contain a hash of the contents of the .mix file.
 - .mix files now have (the same) hash number.

This changes allow different binaries that use the same module compiled in the same way
to share coverage information.

 - Now we use two pipes and a fork rather than use named pipes
 - We use the HPCRIX env var to send the pipe numbers in %d:%d format
 - We now always breakpoint for special events
 - We check to see we are the original process before each breakpoint event
   sub processes are not debugged; this lets us debug the debugger.

 - Added HPCRIX support for passing tracer filename.
 - Added thread tracing support.
 - Cleaned up use of HsFFI.h

This large checkin is the new ghc version of Haskell
Program Coverage, an expression-level coverage tool for Haskell.

Parts:

 - Hpc.[ch] - small runtime support for Hpc; reading/writing *.tix files.
 - Coverage.lhs - Annotates the HsSyn with coverage tickboxes.
  - New Note's in Core,
      - TickBox      -- ticked on entry to sub-expression
      - BinaryTickBox  -- ticked on exit to sub-expression, depending
	       	     -- on the boolean result.

  - New Stg level TickBox (no BinaryTickBoxes, though) 

You can run the coverage tool with -fhpc at compile time. 
Main must be compiled with -fhpc.