force_not_null option support for file_fdw

Hi,

I propose to support force_not_null option for file_fdw too.

In 9.1 development cycle, file_fdw had been implemented with exported
COPY FROM routines, but only force_not_null option has not been　
supported yet.

Originally, in COPY FROM, force_not_null is specified as a list of
column which is not matched against null-string. Now per-column FDW
option is available, so I implemented force_not_null options as boolean
value for each column to avoid parsing column list in file_fdw.

True means that the column is not matched against null-string, and it's
equivalent to specify the column's name in force_not_null option of COPY
FROM. Default value is false.

The patch includes changes for code, document and regression tests. Any
comments/questions are welcome.

Regards,
--
Shigeru Hanada