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>

contrib/pgstattuple/pgstattuple.c

@@ -89,7 +89,7 @@ static Datum
 build_pgstattuple_type(pgstattuple_type *stat, FunctionCallInfo fcinfo)
 {
 #define NCOLUMNS	9
-#define NCHARS		32
+#define NCHARS		314
 
 	HeapTuple	tuple;
 	char	   *values[NCOLUMNS];