Add check for 'file' command which is necessary for 32-bit userspace detection.
auto merge of #5837 : tdaede/rust/fix_file_command, r=catamorphism

Without 'file' it assumes a 32 bit userspace even with a 64 bit kernel, which is incorrect in most cases.
auto merge of #5849 : thestinger/rust/ffi, r=brson

The code samples are xfail'ed because the buildbot won't have `libsnappy.so` and I don't want to add boilerplate to all the snippets (but they're directly from my snappy bindings so I'll just send a pull request whenever they break). It works really well as an example because it's tiny library and lets the caller manage all the buffers.

I think everything stated in the `Interoperability with foreign code` section is accurate but it deserves a thorough double-check :).
tutorial-ffi: add missing word