pgsql: Clean up the code for to_timestamp's conversion of year plus ISO

Log Message:
-----------
Clean up the code for to_timestamp's conversion of year plus ISO day number
to date, as per bug #4702 and subsequent discussion. In particular, make it
work for years specified using AD/BC or CC fields, and fix the test for "no
year specified" so that it doesn't trigger inappropriately for 1 BC (which it
was doing even in code paths that had nothing to do with to_timestamp). I
also did some minor code beautification in the non-ISO-day-number code path.

This area has been busted all along, but because the code has been rewritten
repeatedly, it would be considerable trouble to back-patch. It's such a
corner case that it doesn't seem worth the effort.

Modified Files:
--------------
pgsql/src/backend/utils/adt:
formatting.c (r1.155 -> r1.156)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/formatting.c?r1=1.155&r2=1.156)
timestamp.c (r1.196 -> r1.197)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/timestamp.c?r1=1.196&r2=1.197)