Add 'prompt.*' config options to buildstream.conf

Description

When asking BuildStream to remove a workspace directory, it will pause for a bit and then present the user with an 'Are you sure ... ?' prompt.

Some users prefer not to be interrupted by such prompts, so pave the way to creating options to disable all those that might get in the way.

Follow the example of the advice.* options 'git-config', and create a namespace for these UI options grouped by behaviour, rather than an over-reaching ui.* namespace. In later work perhaps we'll also add advice.* options.

Changes proposed in this merge request:

• Add options to buildstream.conf:
  • prompt.workspace-reset-hard
  • prompt.workspace-close-remove-dir
  • prompt.auto-init
• Expand scheduler.on-error to allow 'terminate', this seems to have been missed by accident.
• Some minor refactoring and doc fixes.

Related to #726 (closed)

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