transformations: Add '--with-patch'.

Suggested by Philippe Swartvagher <philippe.swartvagher@inria.fr>.

* guix/transformations.scm (transform-package-patches): New procedure.
(%transformations): Add it as 'with-patch'.
(%transformation-options, show-transformation-options-help/detailed):
Add '--with-patch'.
* tests/transformations.scm ("options->transformation, with-patch"): New
test.
* doc/guix.texi (Package Transformation Options): Document it.