openthread: platform: Correct handling of closing sockets

In this commit, `net_socket_service_register` is called when a platform
module that has sockets is deinitialized, and it's socket/sockets are
closed.
This commit also handles a case when a module tries to join a multicast
group and a subscription, from another module, is already present.

Also, covered network namespace changes done in #99169

Signed-off-by: Cristian Bulacu <cristian.bulacu@nxp.com>