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
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master

Details
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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(-)