pgsql: Add GetForeignColumnOptions() to foreign.c, and add some documen

Add GetForeignColumnOptions() to foreign.c, and add some documentation.

GetForeignColumnOptions provides some abstraction for accessing
column-specific FDW options, on a par with the access functions that were
already provided here for other FDW-related information.

Adjust file_fdw.c to use GetForeignColumnOptions instead of equivalent
hand-rolled code.

In addition, add some SGML documentation for the functions exported by
foreign.c that are meant for use by FDW authors.

(This is the fdw_helper portion of the proposed pgsql_fdw patch.)

Hanada Shigeru, reviewed by KaiGai Kohei

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/9088d1b96504717fd589ff7eeacc96b6d1c08ead

Modified Files
--------------
contrib/file_fdw/file_fdw.c | 49 +++++--------------
doc/src/sgml/fdwhandler.sgml | 105 +++++++++++++++++++++++++++++++++++++++++
src/backend/foreign/foreign.c | 37 ++++++++++++++-
src/include/foreign/foreign.h | 2 +
4 files changed, 154 insertions(+), 39 deletions(-)