[Ur] request for review of file save ffi extension

[Orchidaceae Phalaenopsis]

Hello,

I've modified the file upload demo slightly to save the file to the
hard disk instead of echoing it back:

* https://github.com/orchid-hybrid/urweb-file/tree/master

In particular I have used ffi handlers to make saving the file a
transactional operation. I've been very careful to follow the manual
on correct use of the ffi but I would really appreciate if people
would look over this and comment on whether it's correct or not:

* https://github.com/orchid-hybrid/urweb-file/blob/master/file.c

I'm worried about not freeing memory that needed to be, double frees,
using memory after free, things like that.

(N.B. This is step 1 of a larger project, I need to save files to a
folder so that they can be processed by some unix tools before the
results are published)

Thank you very much!