Add system start-up files for "guix publish".

Add system start-up files for "guix publish".

* .gitignore: add etc/guix-publish.conf and /etc/guix-publish.service.
* etc/guix-publish.conf.in: New file.
* etc/guix-publish.service.in: New file.
* nix/local.mk (etc/guix-%.service, etc/guix-%.conf): Generalized former
  build-rules for by using patterns.
  (nodist_systemdservice_DATA): Add etc/guix-publish.service, update
  comment.
  (nodist_upstartjob_DATA): Add etc/guix-publish.conf, update comment.
* doc/guix.texi (Invoking guix publish): Add description for enabling
  "guix publish" on host distros using the new files.