Fix portability failure introduced in commits
d2b0b60e7 et al.
I made a frontend fprintf() format use %m, forgetting that that's only
safe in HEAD not the back branches; prior to
96bf88d52 and
d6c55de1f,
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.