Previously, tailscale upgrade was doing the bare minimum for checking
authenticode signatures via `WinVerifyTrustEx`. This is fine, but we can do
better:
* WinVerifyTrustEx verifies that the binary's signature is valid, but it doesn't
determine *whose* signature is valid; tailscale upgrade should also ensure that
the binary is actually signed *by us*.
* I added the ability to check the signatures of MSI files.
* In future PRs I will be adding diagnostic logging that lists details about
every module (ie, DLL) loaded into our process. As part of that metadata, I
want to be able to extract information about who signed the binaries.
This code is modelled on some C++ I wrote for Firefox back in the day. See
https://searchfox.org/mozilla-central/rev/27e4816536c891d85d63695025f2549fd7976392/toolkit/xre/dllservices/mozglue/Authenticode.cpp
for reference.
Fixes#8284
Signed-off-by: Aaron Klotz <aaron@tailscale.com>
