Let file_fdw access COPY FROM PROGRAM

A while back, there was a push to make COPY gzip-aware. That didn't happen,
but COPY FROM PROGRAM did, and it scratches the same itch.

I have a similar need, but with file_fdw foreign tables. I have .csv.gz
files downloaded to the server, but those CSVs have 100+ columns in them,
and in this case I only really care about a half dozen of those columns.
I'd like to avoid:
- the overhead of writing the uncompressed file to disk and then
immediately re-reading it
- writing unwanted columns to a temp/work table via COPY, and then
immediately re-reading them
- multicorn fdw because it ends up making a python string out of all data
cells
- a csv parsing tool like csvtool or mlr, because they output another CSV
which must be reparsed from scratch

Since file_fdw leverages COPY, it seemed like it would be easy to add the
FROM PROGRAM feature to file_fdw. I began asking questions on #postgresql
IRC, only to discover that Adam Gomaa ( akgomaa(at)gmail(dot)com ) had already
written such a thing, but hadn't submitted it. Attached is a small rework
of his patch, along with documentation.

NOTE: The regression test includes unix commands in the program option. I
figured that wouldn't work for win32 systems, so I checked to see what the
regression tests do to test COPY FROM PROGRAM...and I couldn't find any. So
I guess the test exists as a proof of concept that will get excised before
final commit.