Begin working towards Rust 2024 compatibility (microsoft#289)

This PR fixes up the easy issues with Rust 2024 compatibility and
enables the lint for it. However it also scopes out some allows for the
things that need some more work and focus. microsoft#288 is tracking them.

Changes in this PR:

- extern blocks are now unsafe
- `gen` is becoming a reserved keyword, rename variables
- mark unsafe attributes other than linkme's
- resolve the ambiguity of `fn poll` in VmbusSerialDriver

Author: smalis-msft