Added implementation of inter-process atomics.

The inter-process atomics have ipc_ prefixes: ipc_atomic, ipc_atomic_ref
and ipc_atomic_flag. These types are similar to their unprefixed counterparts
with the following distinctions:

- The operations are provided with an added precondition that is_lock_free()
  returns true.
- All operations, including waiting/notifying operations, are address-free,
  so the types are suitable for inter-process communication.
- The new has_native_wait_notify() operation and always_has_native_wait_notify
  static constant allow to test if the target platform has native support for
  address-free waiting/notifying operations. If it does not, a generic
  implementation is used based on a busy wait.
- The new set of capability macros added. The macros are named
  BOOST_ATOMIC_HAS_NATIVE_<T>_IPC_WAIT_NOTIFY and indicate whether address-free
  waiting/notifying operations are supported natively for a given type.

Additionally, to unify interface and implementation of different components,
the has_native_wait_notify() operation and always_has_native_wait_notify
static constant were added to non-IPC atomic types as well. Added
BOOST_ATOMIC_HAS_NATIVE_<T>_WAIT_NOTIFY capability macros to indicate
native support for inter-thread waiting/notifying operations.

Also, added is_lock_free() and is_always_lock_free to atomic_flag.

This commit adds implementation, docs and tests.

test/atomic_api.cpp

@@ -14,7 +14,7 @@
 
 int main(int, char *[])
 {
-    test_flag_api();
+    test_flag_api< boost::atomic_flag >();
 
     test_integral_api< atomic_wrapper, char >();
     test_integral_api< atomic_wrapper, signed char >();