guix package: Re-apply package transformation when upgrading.

* guix/scripts/package.scm (transaction-upgrade-entry)[upgrade]: Add
'transform' parameter.  Pass PKG through it.  Use
'manifest-entry-with-transformations'.
Call 'options->transformation' to get the transformation procedure.
* tests/guix-package.sh: Add 'guix package -u' test.
* tests/packages.scm ("transaction-upgrade-entry, transformation options preserved"):
New test.
* doc/guix.texi (Invoking guix package): Mention that transformations
are preserved across upgrades.
(Package Transformation Options): Likewise.