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/netdev/netdev_register.c

@@ -57,6 +57,7 @@
 
 #include "utils/utils.h"
 #include "igmp/igmp.h"
+#include "mld/mld.h"
 #include "netdev/netdev.h"
 
 #if defined(CONFIG_NET) && CONFIG_NSOCKET_DESCRIPTORS > 0
@@ -418,7 +419,8 @@ int netdev_register(FAR struct net_driver_s *dev, enum net_lltype_e lltype)
 
 #ifdef CONFIG_NET_MLD
       /* Configure the device for MLD support */
-#warning Missing Logic
+
+      mld_devinit(dev);
 #endif
 
       net_unlock();