XXXROUNDUP(): Turns out this fixed Andrew MacIntyre's memory-mgmt

disaster too, so this change is here to stay.  Beefed up the comments
and added some stats Andrew reported.  Also a small change to the
macro body, to make it obvious how XXXROUNDUP(0) ends up returning 0.
See SF patch 578297 for context.

Not a bugfix candidate, as the functional changes here have already
been backported to the 2.2 line (this patch just improves clarity).