Add system start-up files for guix-daemon.

* etc/init.d/guix-daemon.in: New file.
* nix/local.mk (etc/init.d/guix-daemon): New rule.
(nodist_sysvinitservice_DATA): Add etc/init.d/guix-daemon.in .
(CLEANFILES): Add etc/init.d/guix-daemon .
* .gitignore: Add etc/init.d/guix-daemon .