Support for setuid binaries

[Ludovic Courtès]

Hello,

Commit 09e028f adds support for setuid binaries.  Since the store cannot
contain setuid binaries, we use the same technique as NixOS: setuid
binaries are created when booting (or when switching configurations.)

That is, for each setuid program, a hard link or a copy of the
executable is created under /run/setuid-programs and make setuid-root.

You can test it by running:

  ./pre-inst-env guix system vm build-aux/hydra/demo-os.scm

and running for instance ‘ping’ as guest from there.

Ludo’.