guix package: Support package transformation options.

* guix/scripts/package.scm (show-help): Call
'show-transformation-options-help'.
(%options): Append %TRANSFORMATION-OPTIONS.
(process-actions)[transform, transform-entry]: New procedures.
* doc/guix.texi (Invoking guix package): Mention package
transformations.
(Package Transformation Options): Mention 'guix package'.