Repair two recently reported problems:

1) datetime_pl_span() added the seconds field before adding the months
 field.  This lead to erroneous results for e.g.
   select datetime '1999-11-30' + timespan '1 mon - 1 sec';
 Reverse the order of operations to add months first.
2) tm2timespan() did all intermediate math as integer, converting to double
 at the very end. This resulted in hidden overflows when given very large
 integer days, hours, etc. For example,
   select '74565 days'::timespan;
 produced the wrong result. Change code to ensure that doubles are used
 for intermediate calculations.
Thanks to Olivier PRENANT <ohp@pyrenet.fr> and
 Tulassay Zsolt <zsolt@tek.bke.hu> for problem reports and to Tom Lane for
 accurate analyses.