* 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.