Fix more format truncation issues

Fix the warnings created by the compiler warning options
-Wformat-overflow=2 -Wformat-truncation=2, supported since GCC 7.  This
is a more aggressive variant of the fixes in
6275f5d28a, which GCC 7 warned about by
default.

The issues are all harmless, but some dubious coding patterns are
cleaned up.

One issue that is of external interest is that BGW_MAXLEN is increased
from 64 to 96.  Apparently, the old value would cause the bgw_name of
logical replication workers to be truncated in some circumstances.

But this doesn't actually add those warning options.  It appears that
the warnings depend a bit on compilation and optimization options, so it
would be annoying to have to keep up with that.  This is more of a
once-in-a-while cleanup.

Reviewed-by: Michael Paquier <michael@paquier.xyz>

src/backend/commands/explain.c

@@ -3337,10 +3337,11 @@ void
 ExplainPropertyFloat(const char *qlabel, double value, int ndigits,
 					 ExplainState *es)
 {
-	char		buf[256];
+	char	   *buf;
 
-	snprintf(buf, sizeof(buf), "%.*f", ndigits, value);
+	buf = psprintf("%.*f", ndigits, value);
 	ExplainProperty(qlabel, buf, true, es);
+	pfree(buf);
 }
 
 /*