Foreign Data Wrapper fun

I've just started playing with the SQL/MED foreign data wrapper patch, and particularly the file_fdw module. First order of the day has been to get it applied against head, fix the bitrot, and get it building and passing regression tests. That's done so now I'm going to dig deeper and see if I can make it do one or two things I need. People interested in playing along can get the stuff from the sqlmed branch on my github repo.