pgsql: Allow contrib/file_fdw to read from a program, like COPY FROM PR

Allow contrib/file_fdw to read from a program, like COPY FROM PROGRAM.

This patch just exposes COPY's FROM PROGRAM option in contrib/file_fdw.
There don't seem to be any security issues with that that are any worse
than what already exist with file_fdw and COPY; as in the existing cases,
only superusers are allowed to control what gets executed.

A regression test case might be nice here, but choosing a 100% portable
command to run is hard. (We haven't got a test for COPY FROM PROGRAM
itself, either.)

Corey Huinker and Adam Gomaa, reviewed by Amit Langote

Discussion: <CADkLM=dGDGmaEiZ=UDepzumWg-CVn7r8MHPjr2NArj8S3TsROQ(at)mail(dot)gmail(dot)com>

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/8e91e12bc3af85ba2287866669268f6825d2cc03

Modified Files
--------------
contrib/file_fdw/file_fdw.c | 131 +++++++++++++++++++++-----------
contrib/file_fdw/output/file_fdw.source | 6 +-
doc/src/sgml/file-fdw.sgml | 66 +++++++++++-----
3 files changed, 137 insertions(+), 66 deletions(-)