LibCore: Add a way to mark a socket as blocking (or not)

If custom I/O is being done outside CIODevice, we need a way to force blocking sometimes.
This also fixes the default of CLocalSocket to be non-blocking, the same
as CTCPSocket.

Libraries/LibCore/CSocket.h

@@ -23,6 +23,7 @@ public:
     bool send(const ByteBuffer&);
 
     bool is_connected() const { return m_connected; }
+    void set_blocking(bool blocking);
 
     CSocketAddress source_address() const { return m_source_address; }
     int source_port() const { return m_source_port; }