Introduce socket `shutdown()` into connection type, and use it
on normal socket if a fork is active. This allows us to close
client connections when there are child processes sharing the
file descriptors.
Fixes#10077. The reason is that since the `fork()` child is holding
the file descriptors, the `close` in `unlinkClient -> connClose`
isn't sufficient. The client will not realize that the connection is
disconnected until the child process ends.
Let's try to be conservative and only use shutdown when the fork is active.
fac188b49d