net: introduce Sock::WaitMany()

It allows waiting concurrently on more than one socket. Being a
`virtual` `Sock` method it can be overriden by tests.

Will be used to replace `CConnman::SocketEvents()`.

src/test/fuzz/util.h

@@ -72,6 +72,8 @@ public:
 
     bool Wait(std::chrono::milliseconds timeout, Event requested, Event* occurred = nullptr) const override;
 
+    bool WaitMany(std::chrono::milliseconds timeout, EventsPerSock& events_per_sock) const override;
+
     bool IsConnected(std::string& errmsg) const override;
 };