publish: Add '--user' option.

* guix/scripts/publish.scm (show-help): Add --user.
  (%options): Likewise.
  (run-publish-server): Change 'port' parameter to 'socket'.  Pass
  #:socket instead of #:addr and #:port to 'run-server'.  Update caller
  accordingly.
  (open-server-socket, gather-user-privileges): New procedures.
  (guix-publish): Use them.  Force %PRIVATE-KEY and %PUBLIC-KEY early
  on.  Warn when running as root.
* doc/guix.texi (Invoking guix publish): Document --user.