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Among others:
- Fixed Stg->C-- translation of let-no-escapes -- it's important to use the
  right continuation...
- Fixed infinite recursion in X86 backend (shortcutJump mishandled infinite loops)
- Fixed yet another wrong calling convention -- primops take args only in vanilla regs,
  but they may return results on the stack!
- Removed StackInfo from LGraph and Block -- now in LastCall and CmmZ
- Updated avail-variable and liveness code

We want the internal format to contain LFs only. This makes it easier to work
with the doc strings for clients of the GHC API.

Digits already have a couple of meanings in z-encoding (Z3T is a tuple,
and z123U is a unicode character), so we encode digits as unicode
characters to avoid trying to squeeze in another meaning.

Also removed a little GHC < 6.2 compatibility hack.

 - If each basic block doesn't end with a jump then the register
   liveness determinator will get the cross-block liveness info
   wrong, resulting in a bad allocation.

- Package dph ist a meta package that contains a number of subpackages inside.
- This patch adds two targets to clean and remake all subpackages with a
  single command.

In particular, when building dph with the stage2 compiler, we want to
register it in the main package.conf.

The API is the same (for now).  The new implementation has the
capability to define signal handlers that have access to the siginfo
of the signal (#592), but this functionality is not exposed in this
patch.

#2451 is the ticket for the new API.

The main purpose of bringing this in now is to fix race conditions in
the old signal handling code (#2858).  Later we can enable the new
API in the HEAD.

Implementation differences:

 - More of the signal-handling is moved into Haskell.  We store the
   table of signal handlers in an MVar, rather than having a table of
   StablePtrs in the RTS.

 - In the threaded RTS, the siginfo of the signal is passed down the
   pipe to the IO manager thread, which manages the business of
   starting up new signal handler threads.  In the non-threaded RTS,
   the siginfo of caught signals is stored in the RTS, and the
   scheduler starts new signal handler threads.