Port "client_port: Make all data members private" from yuzu (#4064)

* client_port: Make all data members private

These members don't need to be entirely exposed, we can instead expose
an API to operate on them without directly needing to mutate them

We can also guard against overflow/API misuse this way as well, given
active_sessions is an unsigned value.

* make the condition an assert

src/core/hle/kernel/server_session.cpp

@@ -20,7 +20,7 @@ ServerSession::~ServerSession() {
 
     // Decrease the port's connection count.
     if (parent->port)
-        parent->port->active_sessions--;
+        parent->port->ConnectionClosed();
 
     // TODO(Subv): Wake up all the ClientSession's waiting threads and set
     // the SendSyncRequest result to 0xC920181A.