pgsql: Make extract() do something more reasonable with infinite dateti

Make extract() do something more reasonable with infinite datetimes.

Historically, extract() just returned zero for any case involving an
infinite timestamp[tz] input; even cases in which the unit name was
invalid.  This is not very sensible.  Instead, return infinity or
-infinity as appropriate when the requested field is one that is
monotonically increasing (e.g, year, epoch), or NULL when it is not
(e.g., day, hour).  Also, throw the expected errors for bad unit names.

BACKWARDS INCOMPATIBLE CHANGE

Vitaly Burovoy, reviewed by Vik Fearing

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/647d87c56ab6da70adb753c08d7cdf7ee905ea8a

Modified Files
--------------
doc/src/sgml/func.sgml             |   11 ++
src/backend/utils/adt/timestamp.c  |  111 +++++++++++++++--
src/test/regress/expected/date.out |  242 ++++++++++++++++++++++++++++++++++++
src/test/regress/sql/date.sql      |   53 ++++++++
4 files changed, 405 insertions(+), 12 deletions(-)