pgsql: Provide a FORCE NULL option to COPY in CSV mode.

Provide a FORCE NULL option to COPY in CSV mode.

This forces an input field containing the quoted null string to be
returned as a NULL. Without this option, only unquoted null strings
behave this way. This helps where some CSV producers insist on quoting
every field, whether or not it is needed. The option takes a list of
fields, and only applies to those columns. There is an equivalent
column-level option added to file_fdw.

Ian Barwick, with some tweaking by Andrew Dunstan, reviewed by Payal
Singh.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/3b5e03dca2afea7a2c12dbc8605175d0568b5555

Modified Files
--------------
contrib/file_fdw/data/text.csv | 9 ++--
contrib/file_fdw/file_fdw.c | 71 +++++++++++++++++++++++------
contrib/file_fdw/input/file_fdw.source | 17 ++++++-
contrib/file_fdw/output/file_fdw.source | 41 +++++++++++++----
doc/src/sgml/file-fdw.sgml | 21 +++++++--
doc/src/sgml/ref/copy.sgml | 19 +++++++-
src/backend/commands/copy.c | 74 +++++++++++++++++++++++++++++--
src/backend/parser/gram.y | 4 ++
src/test/regress/expected/copy2.out | 48 ++++++++++++++++++++
src/test/regress/sql/copy2.sql | 39 ++++++++++++++++
10 files changed, 308 insertions(+), 35 deletions(-)