std: Close TcpListener with closesocket()

This may have inadvertently switched during the runtime overhaul, so this
switches TcpListener back to using sockets instead of file descriptors. This
also renames a bunch of variables called `fd` to `socket` to clearly show that
it's not a file descriptor.

Closes #19333
Merge remote-tracking branch 'alexcrichton/issue-19333' into oibit2