pgsql: Fix interval division and multiplication, before: test=> select

Log Message:
-----------
Fix interval division and multiplication, before:

test=> select '4 months'::interval / 5;
?column?
---------------
1 mon -6 days
(1 row)

after:

test=> select '4 months'::interval / 5;
?column?
----------
24 days
(1 row)

The problem was the use of rint() to round, and then find the remainder,
causing the negative values.

Modified Files:
--------------
pgsql/src/backend/utils/adt:
timestamp.c (r1.132 -> r1.133)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/timestamp.c.diff?r1=1.132&r2=1.133)