Foreign-declarative module

[KAction]

Hello!

Below is set of patches, that (1-20) implement new module (system
foreign declarative), that simplifies writing bindings to foreign C
functions and provides documentation to it. This module is extracted
from abandoned guile-bash project.

Rest 5 patches (21-25) introduce module (ice-9 xattr), that provide
Scheme interface to extended filesystem attributes, and is
proof-of-usefulness of (system foreign declarative) module.

There actually should also be patch 26, that write documentation for xattr
module, but

 * I am not sure, in which file under docs/ref to put it in
 * I think, what is already done deserves review.

If you wish, I could collapse these patches. Thanks for your review time.

PS. Please, keep me in CC, I am not subscribed to list.