Extend and rename wxSocketImpl::UnblockAndRegisterWithEventLoop()

In addition to unblocking and registering the socket, also support using
this function to make the socket blocking and unregistering it from the
event loop, if its flags include wxSOCKET_BLOCK.

This was already half-done by wxMSW, which took wxSOCKET_BLOCK presence
into account in its implementation, but not by the Unix implementation.
Now do it under all platforms, as this will be useful for switching a
previously non-blocking socket to blocking mode.

Finally, rename the function to better reflect what it really does.

See #12886.

include/wx/private/socket.h

@@ -323,9 +323,11 @@ private:
     // called by Close() if we have a valid m_fd
     virtual void DoClose() = 0;
 
-    // put this socket into non-blocking mode and enable monitoring this socket
-    // as part of the event loop
-    virtual void UnblockAndRegisterWithEventLoop() = 0;
+    // Update the socket depending on the presence or absence of wxSOCKET_BLOCK
+    // in GetSocketFlags(): if it's present, make the socket blocking and
+    // ensure that we don't get any asynchronous event for it, otherwise put
+    // it into non-blocking mode and enable monitoring it in the event loop.
+    virtual void UpdateBlockingState() = 0;
 
     // check that the socket wasn't created yet and that the given address
     // (either m_local or m_peer depending on the socket kind) is valid and
@@ -350,7 +352,7 @@ private:
     }
 
     // apply the options to the (just created) socket and register it with the
-    // event loop by calling UnblockAndRegisterWithEventLoop()
+    // event loop by calling UpdateBlockingState()
     void PostCreation();
 
     // update local address after binding/connecting