pgsql: Properly round months into days and into seconds for interval

Log Message:
-----------
Properly round months into days and into seconds for interval
multiplication/division queries like select '41 mon 10:00:00'::interval
/ 10 as "pos".

Report from Michael Glaesemann

Modified Files:
--------------
    pgsql/src/backend/utils/adt:
        timestamp.c (r1.165 -> r1.166)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/timestamp.c.diff?r1=1.165&r2=1.166)
    pgsql/src/test/regress/expected:
        interval.out (r1.15 -> r1.16)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/interval.out.diff?r1=1.15&r2=1.16)