Implement information schema interval_type columns

Also correct reporting of interval precision when field restrictions
are specified in the typmod.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/0527a454ec1801358a1f6d873ae48dcc89a217cf

Modified Files
--------------
doc/src/sgml/information_schema.sgml       |   30 +++++++++++++++++++++++++--
src/backend/catalog/information_schema.sql |   28 ++++++++++++++++++++++---
2 files changed, 51 insertions(+), 7 deletions(-)

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