make isc_nmsocket_{attach,detach}{} functions private

make isc_nmsocket_{attach,detach}{} functions private

there is no need for a caller to reference-count socket objects.
they need tto be able tto close listener sockets (i.e., those
returned by isc_nm_listen{udp,tcp,tcpdns}), and an isc_nmsocket_close()
function has been added for that. other sockets are only accessed via
handles.

(cherry picked from commit 9e740cad21972194c063f9e471a7ba7d910ee01f)