package: Allow multiple '--manifest' options.

* guix/scripts/package.scm (manifest-action): Remove.
(%actions): Remove it.
(load-manifest): New procedure.
(process-actions): Handle 'manifest' options.  Define 'files' from
'manifest' options.  Define 'manifest' based on FILES.  Define 'trans'
to represent the final transaction.
* tests/guix-package.sh: Test it.
* doc/guix.texi (Invoking guix package): Mention