use_bbi fixes

[kyleam]

This series fixes a few use_bbi issues. These were prompted/surfaced by Windows testing (see gh-576), but most of these are also problems outside of Windows.

Main issues fixed:

• "set bbr.bbi_exe_path or add to PATH" message was shown even if caller aborted

• "set bbr.bbi_exe_path or add to PATH" message misfired on valid bbr.bbi_exe_path if it didn't exactly match the normalizePath() output

• "set bbr.bbi_exe_path or add to PATH" did not fire if the paths directory was in PATH but overshadowed

• relative path like use_bbi("bin/bbi") triggered a dev error

• ensure target path on Windows ends with ".exe"

Closes #576

(I'll have one followup PR that uses this as a base. edit: that's gh-581)

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todo:

• [x] final round of interactive testing on windows
• [x] adjust for compatibility with mpn oldest (failing build)

[kyleam]

prevent indented newline from being trimmed

Don't think this is going to trip anyone up, but that should say "intended" not "indented" :x