Add sockets_priv.h header

This commit introduces a sockets_priv.h header for socket API internal
implementations intended to be used by sockets API C files, but not
applications

This commit moves struct lwip_setgetsockopt_data to the private header
because this is not part of the public sockets API, but needs to be
shared between sockets.c and memp.c

This header lays ground work for sharing other internal sockets types
/macros between API files (sockets.c and if_api.c)

src/core/memp.c

@@ -63,6 +63,7 @@
 #include "lwip/priv/tcpip_priv.h"
 #include "lwip/priv/api_msg.h"
 #include "lwip/sockets.h"
+#include "lwip/priv/sockets_priv.h"
 #include "lwip/netifapi.h"
 #include "lwip/etharp.h"
 #include "lwip/igmp.h"