Fix portability failure introduced in commits d2b0b60e et al.

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.

src/common/psprintf.c

@@ -115,7 +115,8 @@ pvsnprintf(char *buf, size_t len, const char *fmt, va_list args)
 #ifndef FRONTEND
 		elog(ERROR, "vsnprintf failed: %m with format string \"%s\"", fmt);
 #else
-		fprintf(stderr, "vsnprintf failed: %m with format string \"%s\"\n", fmt);
+		fprintf(stderr, "vsnprintf failed: %s with format string \"%s\"\n",
+				strerror(errno), fmt);
 		exit(EXIT_FAILURE);
 #endif
 	}