sockets: task #14247, add CMSG and IP_PKTINFO

This commit adds CMSG infrastructure (currently used with recvmsg) and
the IP_PKTINFO socket option.

In order to use IP_PKTINFO, set LWIP_NETBUF_RECVINFO to 1

Unit test is added to verify this feature

test/unit/lwipopts.h

@@ -40,6 +40,7 @@
 #define LWIP_NETCONN                    !NO_SYS
 #define LWIP_SOCKET                     !NO_SYS
 #define LWIP_NETCONN_FULLDUPLEX         LWIP_SOCKET
+#define LWIP_NETBUF_RECVINFO            1
 #define LWIP_HAVE_LOOPIF                1
 #define TCPIP_THREAD_TEST