Add `--max-silent-time' to `guix build' and `guix package'.

* guix/scripts/build.scm (%default-options): Add default
  `max-silent-time' value.
  (show-help, %options):  Add `--max-silent-time'.
  (guix-build): Pass `max-silent-time' to `set-build-options'.
* guix/scripts/package.scm (%default-options): Add default
  `max-silent-time' value.
  (show-help, %options):  Add `--max-silent-time'.
  (guix-package): Pass `max-silent-time' to `set-build-options'.
* guix/ui.scm (string->number*): New procedure.
* tests/derivations.scm ("build-expression->derivation and
  max-silent-time"): New test.
* doc/guix.texi (Invoking guix package, Invoking guix build): Document
  `--max-silent-time'.