pgsql: Fix portability failure introduced in commits d2b0b60e7 et al.

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.

Branch
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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/b8fa4c1626190142d2e98155f9df99f80fe01cef

Modified Files
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src/common/psprintf.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)