installer: Ask for user password and initialize /etc/shadow.

installer: Ask for user password and initialize /etc/shadow.

Partly fixes <https://bugs.gnu.org/35399>.

* gnu/installer/user.scm (<user>)[password]: New field.
* gnu/installer/final.scm (%seed): New variable.
(integer->alphanumeric-char, random-string)
(create-user-database): New procedures.
(install-system): Call 'create-user-database'.
* gnu/installer/newt/final.scm (run-install-shell): Add #:users and pass
it to 'install-system'.
(run-final-page): Pass #:users to 'run-install-shell'.
* gnu/installer/newt/user.scm (run-user-add-page): Add password entry.
Pass its result as the 'password' field of <user>.