Overhaul and vastly simplify service registration process.

[mhammond]

No description provided.

[mhammond]

After things like #1795, #1811, #1871 and #1840, I decided that really the service registration code was plain broken and lots of old 20 year cruft that confused the issue, and that complicated warning just made things worse. So I just removed most of that and tried to keep it simple - if pythonservice.exe isn't in a good place,at service registration time we just move it to sys.exec_prefix and use it from there.

It works for me in a vanilla pypi install of pywin32, even without the post-install script having been run, but it should work even when it has and even when installed via a .exe

@Fnordbert, @Wissperwind, @MedvedevSA, @lucasg you've all been burnt by this in the issues above, so if any of you have time, I'd love some feedback about whether it works for you. Artifacts can be found by selecting the "actions" tab for the project, finding the most recent runs for this PR, then selecting "artifacts" (it's a bit hidden, let me know if you need help finding them).

[Fnordbert]

Sorry for the slow response. I will have a closer look next time I get to work on my service. (which could still be a while)