installer: Turn passwords into opaque records....* gnu/installer/user.scm (<secret>, secret?, make-secret,
secret-content): Add opaque <secret> record that boxes its contents,
with a custom printer that doesn't display anything.
* gnu/installer/newt/user.scm (run-user-add-page, run-user-page): Box
it.
* gnu/installer/final.scm (create-user-database): Unbox it.
Signed-off-by: Mathieu Othacehe <othacehe@gnu.org>