pgsql: Make the file_fdw validator check that a filename option has bee

Make the file_fdw validator check that a filename option has been provided.

This was already a runtime failure condition, but it's better to check
at validation time if possible.  Lightly modified version of a patch
by Shigeru Hanada.

Branch
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/f8bd267d35503109255c99eae96c86ba8ba1b56f

Modified Files
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contrib/file_fdw/file_fdw.c             |   18 +++++++++++++++---
contrib/file_fdw/input/file_fdw.source  |    1 +
contrib/file_fdw/output/file_fdw.source |    2 ++
3 files changed, 18 insertions(+), 3 deletions(-)