#include "RtsUtils.h" for barf prototype

Fix bogosity in implementation of ACQUIRE_LOCK/RELEASE_LOCK on Win32.
These functions were essentially doing nothing, due to a missing
dereference on the argument.

I've rewritten them as inlines (to catch type errors) and added some
checking of the return values, which should help catch errors like
this in the future.

debug versions of ACQUIRE_LOCK() and RELEASE_LOCK() macros

win32: make it compile

- renamed MutexVar to Mutex, CondVar to Condition.
- added yieldThread().
- simplified/specialised type of entry point to new (native) threads;
  now takes no args & returns no result.

Win32 implementation, first pass.

First steps towards implementing better interop between
Concurrent Haskell and native threads.

- factored out Capability handling into a separate source file
  (only the SMP build uses multiple capabilities tho).
- factored out OS/native threads handling into a separate
  source file, OSThreads.{c,h}. Currently, just a pthreads-based
  implementation; Win32 version to follow.
- scheduler code now distinguishes between multi-task threaded
  code (SMP) and single-task threaded code ('threaded RTS'),
  but sharing code between these two modes whenever poss.

i.e., just a first snapshot; the bulk of the transitioning code
remains to be implemented.