Prepare patch for ticket 5129 for merging.

- Preserve old eventdns code.
- Add function to close sockets cross-platform, without accounting.
- Add changes/ file.
Actually use the cloexec argument in the !defined(SOCK_CLOEXEC) case
Simple unit test for tor_open_socket_with_extensions
Split the socketpair replacement code into its own function for testing
Unit test for tor_{ersatz_,}socketpair.

This catches tor_accept as well.
Fixed "unused parameter cloexec" warnings on windows