store: Add #:select? parameter to 'add-to-store'.

* guix/store.scm (write-arg): Remove 'file' case.
(true): New procedure.
(add-to-store): Add #:select? parameter and honor it.  Use hand-coded
stub instead of 'operation'.
(interned-file): Add #:select? parameter and honor it.
* doc/guix.texi (The Store Monad): Adjust 'interned-file' documentation
accordingly.