CLEANUP: dns: centralize nameserver transport cleanup

CLEANUP: dns: centralize nameserver transport cleanup

Several resolver configuration error paths open-code nameserver
transport cleanup. They also assume that a nameserver owns either a
datagram transport or a stream transport, but never both.

Add dns_nameserver_deinit() and use it from these error paths. The
helper releases each transport's configuration-time allocations
independently and clears its pointer. No functional change is intended.