guix build: Add '--with-debug-info'.

guix build: Add '--with-debug-info'.

* guix/scripts/build.scm (transform-package-with-debug-info): New
procedure.
(%transformations): Add 'with-debug-info'.
(%transformation-options, show-transformation-options-help): Add
'--with-debug-info'.
* tests/scripts-build.scm ("options->transformation, with-debug-info"):
New test.
* doc/guix.texi (Package Transformation Options): Document
'--with-debug-info'.
(Installing Debugging Files): Introduce sections.  Remove bit about
eventual "opt-out" since this is not happening.  Document
'--with-debug-info' under "Rebuilding with Debugging Info".