Add pg_file_sync() to adminpack

Hi,

I'd like to propose to add pg_file_sync() function into contrib/adminpack.
This function fsyncs the specified file or directory named by its argument.
IMO this is useful, for example, when you want to fsync the file that
pg_file_write() writes out or that COPY TO exports the data into,
for durability. Thought?

Regards,

-- 
Fujii Masao