config: rename allow_archive to allow-archive
As part of ConfigConsolidationPlan [1], rename the option according to
the new UI guidelines [2] and add an alias for backward compatibility.
[1]: https://www.mercurial-scm.org/wiki/ConfigConsolidationPlan
[2]: https://www.mercurial-scm.org/wiki/UIGuideline#adding_new_options
(binary:application/octet-stream)