engine: catchall handler is shadowed by specific handlers

This allows using `Engine.Register` and `Engine.RegisterCatchall` on the
same engine without the catchall hiding all other handlers.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Engine: empty job names are illegal, catchall or not

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Merge pull request #5538 from shykes/pr_out_engine_catchall_handler_is_shadowed_by_specific_handlers