Clean up NetAccept

[moonchen]

• Give sensible names to the three different accept code paths (dedicated, per-thread, periodic)
• Put common code into functions - vc initialization, send/recv buf settings
• Make QUIC UDP code path separate from TCP

[moonchen]

During the course of this cleanup, I found out that the different code paths have small differences in how they set up socket and UnixNetVConnection options. I don't think those are intentional differences, so they have been unified in this PR.

[github-actions[bot]]

This pull request has been automatically marked as stale because it has not had recent activity. Marking it stale to flag it for further consideration by the community.