Squashed commit of the following:

net/mld:  The MLD logic now compiles and is much less toxic.  It still is not a proper MLD implementation:  (1) It is basically a port of IGMP, tweaked to work with IPv6 and ICMPv6 MLD messages, (2) it needs a proper analysis and comparison with RFC 3810, and (3) it is completely untested.  For this reason, it will remain EXPERIMENTAL for some time.
    net/mld:  Add some missing macros, more fixes related to IPv6 vs IPv4 types,
    net/mld:  More compilation cleaning.  Most fixups for IPv6 vs IPv4 types.
    net/mld:  Hook crudely converted .c files into build system and resolve a few of the many, many compilation/design problems.
    net/mld:  Add support for MLD statistics.
    net/mld:  Hook in MLD poll and packet transmission logic.
    net/mld:  Change references to IPv4 definitions to IPv6 definitions; Remove mld_input() since MLD piggybacks on ICMPv6 input.  Add functions to catch MLD messages dispatched by ICMPv6 input logic.
    net/mld:  As a starting point, copy all net/igmp/*.c files to net/mld/. and change all occurrences of igmp (or IGMP) to mld (or MLD).
    net/mld:  More compilation cleaning.  Most fixups for IPv6 vs IPv4 types.
    net/mld:  Hook crudely converted .c files into build system and resolve a few of the many, many compilation/design problems.
    net/mld:  Add support for MLD statistics.
    net/mld:  Hook in MLD poll and packet transmission logic.
    net/mld:  Change references to IPv4 definitions to IPv6 definitions; Remove mld_input() since MLD piggybacks on ICMPv6 input.  Add functions to catch MLD messages dispatched by ICMPv6 input logic.
    net/mld:  As a starting point, copy all net/igmp/*.c files to net/mld/. and change all occurrences of igmp (or IGMP) to mld (or MLD).

net/devif/devif_poll.c

@@ -57,8 +57,9 @@
 #include "bluetooth/bluetooth.h"
 #include "ieee802154/ieee802154.h"
 #include "icmp/icmp.h"
-#include "icmpv6/icmpv6.h"
 #include "igmp/igmp.h"
+#include "icmpv6/icmpv6.h"
+#include "mld/mld.h"
 #include "ipforward/ipforward.h"
 #include "sixlowpan/sixlowpan.h"
 
@@ -421,6 +422,36 @@ static inline int devif_poll_igmp(FAR struct net_driver_s *dev,
 }
 #endif /* CONFIG_NET_IGMP */
 
+/****************************************************************************
+ * Name: devif_poll_mld
+ *
+ * Description:
+ *   Poll all MLD connections for available packets to send.
+ *
+ * Assumptions:
+ *   This function is called from the MAC device driver with the network
+ *   locked.
+ *
+ ****************************************************************************/
+
+#ifdef CONFIG_NET_MLD
+static inline int devif_poll_mld(FAR struct net_driver_s *dev,
+                                 devif_poll_callback_t callback)
+{
+  /* Perform the MLD TX poll */
+
+  mld_poll(dev);
+
+  /* Perform any necessary conversions on outgoing ICMPv6 packets */
+
+  devif_packet_conversion(dev, DEVIF_ICMP6);
+
+  /* Call back into the driver */
+
+  return callback(dev);
+}
+#endif /* CONFIG_NET_MLD */
+
 /****************************************************************************
  * Name: devif_poll_udp_connections
  *
@@ -626,6 +657,15 @@ int devif_poll(FAR struct net_driver_s *dev, devif_poll_callback_t callback)
 
   if (!bstop)
 #endif
+#ifdef CONFIG_NET_MLD
+    {
+      /* Check for pending MLD messages */
+
+      bstop = devif_poll_mld(dev, callback);
+    }
+
+  if (!bstop)
+#endif
 #ifdef NET_TCP_HAVE_STACK
     {
       /* Traverse all of the active TCP connections and perform the poll