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'.