pgsql: Fix handling of BC years in to_date/to_timestamp.

Fix handling of BC years in to_date/to_timestamp.

Previously, a conversion such as
to_date('-44-02-01','YYYY-MM-DD')
would result in '0045-02-01 BC', as the code attempted to interpret
the negative year as BC, but failed to apply the correction needed
for our internal handling of BC years. Fix the off-by-one problem.

Also, arrange for the combination of a negative year and an
explicit "BC" marker to cancel out and produce AD. This is how
the negative-century case works, so it seems sane to do likewise.

Continue to read "year 0000" as 1 BC. Oracle would throw an error,
but we've accepted that case for a long time so I'm hesitant to
change it in a back-patch.

Per bug #16419 from Saeed Hubaishan. Back-patch to all supported
branches.

Dar Alathar-Yemen and Tom Lane

Discussion: https://postgr.es/m/16419-d8d9db0a7553f01b@postgresql.org

Branch
------
REL9_6_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/19e7982681df74e20aa062d5605cb0f7e04c5d51

Modified Files
--------------
doc/src/sgml/func.sgml | 14 ++++++++++--
src/backend/utils/adt/formatting.c | 7 ++++--
src/test/regress/expected/horology.out | 39 ++++++++++++++++++++++++++++++++++
src/test/regress/sql/horology.sql | 11 ++++++++++
4 files changed, 67 insertions(+), 4 deletions(-)