Fix nsupdate hang when processing a large update

Fix nsupdate hang when processing a large update

The root cause is the fix for CVE-2024-0760 (part 3), which resets
the TCP connection on a failed send. Specifically commit
4b7c61381f186e20a476c35032a871295ebbd385 stops reading on the socket
because the TCP connection is throttling.

When the tcpdns_send_cb callback thinks about restarting reading
on the socket, this fails because the socket is a client socket.
And nsupdate is a client and is using the same netmgr code.

This commit removes the requirement that the socket must be a server
socket, allowing reading on the socket again after being throttled.