Issue title: Suggestion for the Boost Build debugger
 Issue body: My suggestion is simple: a 'break' command that lists the current breakpoints which the user has set, and whether each breakpoint is enabled or disabled. This will greatly help the user to set, delete, enable, and disable breakpoints while he is using the Boost Build debugger.
 Comments: 
 Comment 0: Thank you for your contributions. Main development of B2 has moved to https://github.com/bfgroup/b2
This issue has been automatically marked as "transition" to indicate the potential for needing transition to the new B2 development project.

{'source': 'GithubIssues', 'repo_name_with_owner': 'boostorg/build', 'issue_id': 473685073, 'repo_stars': 237.0}