Replace WinVerifyTrust with a cross-platform signature verification API

[woodruffw]

The only thing blocking winchecksec builds for non-Windows is a single API: WinVerifyTrust.

We should try to remove usage of it in favor of an open-source implementation, of which there should be a few (since Authenticode is basically PKCS#7).

Some potential starting points:

• WINE has a WinVerifyTrust implementation
• Technical documentation on Authenticode from MS: http://www.microsoft.com/whdc/winlogo/drvsign/Authenticode_PE.mspx

cc @haxmeadroom @ekilmer

[woodruffw]

The WINE WinVerifyTrust impl. doesn't look especially useful: https://source.winehq.org/source/dlls/wintrust/wintrust_main.c

[woodruffw]

As an update: I looked a bit into this last night, and it looks like it'd be pretty easy to do with pe-parse and some OpenSSL. My goal is to make a small cross-platform WinVerifyTrust replacement library in the next week or so.

[woodruffw]

Update: I have a working prototype for this.

[woodruffw]

Update: smolverify can do this.