I made a frontend fprintf() format use %m, forgetting that that's only
safe in HEAD not the back branches; prior to 96bf88d5 and d6c55de1,
it would work on glibc platforms but not elsewhere.  Revert to using
%s ... strerror(errno) as the code did before.

We could have left HEAD as-is, but for code consistency across branches,
I chose to apply this patch there too.

Per Coverity and a few buildfarm members.