Integration of the Nix daemon

[Ludovic Courtès]

Hi,

I just pushed a new ‘nix-integration’ branch.  The goal is to import the
subset of Nix that we need to run the daemon in Guix itself, so that 0.1
will hopefully be self-contained.

The branch adds a script to synchronize the code with upstream Nix
(think ‘gnulib-update’.)  It extends the build system to build that C++
code, and adds a little bit of glue code to put it all together.

There’s still some functionality missing.  In Nix it’s provided by a
bunch of hairy Perl scripts that nix-daemon calls, but I’d like to
rework that part in an attempt to honor a zero-Perl policy.  :-)

Comments welcome!

Ludo’.