ui: separate option to show prompt echo, enabled only in tests (issue4417) The problem in commandserver was addressed by 524b786bd54f, but it is tricky to reuse ui.nontty option to disable echo back. Instead, this patch introduces new option to enable echoing of prompt response. Prompt echoing is changed to be off by default, which should avoid possible breakage of output parsing in user scripts. For now, this option is undocumented because it exists for internal use.