pgsql: Re-allow input of Julian dates prior to 0001-01-01 AD.

Re-allow input of Julian dates prior to 0001-01-01 AD.

This was unintentionally broken in 8.4 while tightening up checking of
ordinary non-Julian date inputs to forbid references to "year zero".
Per bug #5672 from Benjamin Gigot.

Branch
------
REL9_0_STABLE

Details
-------
http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=6f664c1af4ebb8d4f131af04d5762ac6f720b70b

Modified Files
--------------
src/backend/utils/adt/datetime.c       |   37 +++++++++++++++++++++----------
src/test/regress/expected/horology.out |   13 +++++++++++
src/test/regress/sql/horology.sql      |    3 ++
3 files changed, 41 insertions(+), 12 deletions(-)