include/nuttx/net/ipv6ext.h: Fix some terminology: Hop-by-hop, not Hop-to-hop or Hop2hop.
net/devif/ipv6_input.c and net/icmpv6/icmpv6_input.c: Add logic to skiip over the variable number of IPv6 extension headers that may be present between the IPv6 header and the transport layer header. The extension headers are simply ignored. This is necessary because with MLD, certain incoming messages may have, at a mimimum, a Router Alert Hop-by-hop extension header.
net/inet/ipv6_setsockopt.c: Implement the IPV6_JOIN_GROUP and IPV6_LEAVE_GROUP socket options.
