Thanks to both of you. I installed the attached into Savannah master 
coreutils. It implements the suggestion, except that later I noticed 
that EMLINK and ETXTBSY can join the throng.

At some point it might make sense to scan for other direct or indirect 
calls to renameat, renameat2, and linkat, where the diagnostic could be 
improved if it's known to refer to the destination.