Give an entry for github.com the GitHub service under any name

[czpython]

A caller can now get git and gh for a secret without the handle github. A custom entry whose host is github.com resolves to the built-in github service: GH_TOKEN, the API and upload hosts with a bearer, and github.com with Basic.

• catalog.service reads BUILT_IN_HOSTS before it builds a custom service. The entry's own auth_variable, auth_header, and auth_prefix are not used for such a host.
• The Docker Sandboxes provider chooses the github secret of sbx from the resolved service, not from the entry's handle, so an entry under another handle also gets it.
• An entry with the handle github and no host resolves as before.