Clean up the code for to_timestamp's conversion of year plus ISO day number

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.