scheme port

[Marc Espie <espie_(_at_)_schutzenberger_(_dot_)_liafa_(_dot_)_jussieu_(_dot_)_fr>]

I've just added a scheme port to our collection. I got surprised we did
not have one, and it proved to be reasonably easy.

All the basic features seem to work okay. I haven't really checked dynamic
loading yet (I'll know soon enough, as there is an xscm interface along
the way).

I'm not really a mad lisper, so some details may need some improvement.

- split the slib in a separate package, so that other ports can use it ?
There are probably some fun scripts to add, so that library-vicinity
and implementation-vicinity are set up correctly.

- some glue is probably needed to compile other third-party packages.

- there is at least one compiler (hobbit) available.

- I don't use emacs. Neither the debugger nor the emacs interface have been
added yet.

Right now, the port is very close to the FreeBSD port... things may diverge
later.