update for r104360/104361: event subscribers might still need the zodb, so `clear` must not be called before `notify` as otherwise `self._held=None` might close the connection (refs LP #414757)

Fix HelpSys to work with zope.interface.Interface as it did before the deprecated methods of scarecrow Interface.Interface were removed.

- Cover use on Windows
- Cover buildout-based instances

- correct capitalisation of "command" reg value name, not that it matters ;-)
- add a windebug command to help when debugging windows service behaviour
- refactor do_start: nt_svcutils.service.Service sets ZMANAGED, so its not needed on Windows.

- re-installed some comments that Jeremy obfuscated
- renamed the crazily named "redirectCaptureThread" to a name that matches what it does
- highlight the likely cause of #443005: 

The magic named event is no longer found even though Zope is still running.
I suspect this is because runzope is now a setuptools .exe.

  (this means that buildout instances work on Windows too :-) )
- remove a few stray comments and old unneeded code from nt_svcutils/service.py

Easier to axe this file than give such a short list compared with the huge number of people who've since contributed.

I haven't changed Makefile to use this sphinx-build in case is screws with whatever is used on docs.zope.org.

Backport removal of experimental support for configuring the Twisted HTTP server as an alternative to ``ZServer``.