kernel: atomic_*() functions rework

* No need for the atomically changed variables to be declared as
   volatile.
 * Drop support for atomically getting and setting unaligned data.
 * Introduce atomic_get_and_set[64]() which works the same as
   atomic_set[64]() used to. atomic_set[64]() does not return the
   previous value anymore.

headers/private/package/TempfileManager.h

@@ -31,7 +31,7 @@ private:
 
 private:
 			BDirectory			fBaseDirectory;
-			vint32				fNextNumber;
+			int32				fNextNumber;
 };