accept/recv/send kept the socket RwLock during the syscall. After
fork, the child copied that held lock, so _ResourceSharer._afterfork
hung in listener.close().
Unskip fork connection pickling, pool task ref release, and spawn
thread-pool terminate. Add extra_tests for fork+accept close and
fork after DupFd.
Assisted-by: Grok:4.6
