LLFrameTimer review and resulting changes.

This makes the class API a bit more sane, although
only a bit, because I had to leave it working with
possibly new code merged in from LL: the API can't
really change. I also removed some unused code.

While reviewing how LLFrameTimer worked however,
I did find a few instances where things where broken:

* sFrameDeltaTime wasn't correctly updated (more
  than once per frame and therefore erratic). This
  only influenced scrolling speed, but still.
* While dragging an inventory item, the scroll
  arrows of a tab container didn't work
  (LLTabContainer::handleDragAndDrop).
* Map zoom interpolation was broken (it interpolated
  between A and B, but used the already updated
  interpolation for A the next frame...
  (added mCurZoomValInterpolationStart).

indra/llvfs/llpidlock.cpp

@@ -167,7 +167,7 @@ bool LLPidLockFile::requestLock(LLNameTable<void *> *name_table, bool autosave,
 
 	if (!mWaiting)				//Not presently waiting to save.  Queue up.
 	{
-		mTimer.resetWithExpiry(timeout);
+		mTimer.reset(timeout);
 		mWaiting=TRUE;
 	}