This commit adds support for semi-standard IPPROTO_ICMP6 sockets.  This is a replacement for the non-standard ICMPv6 ping support that violated the portable POSIX OS interface.

Squashed commit of the following:

    net/icmpv6: IPPROT_ICMP6 socket logic now builds without error.
    net/icmpv6:  Add support for read-ahead and poll(). Initial commit is just cloned from ICMP with the appropriate name changes.
    configs/:  All defconfig filess that include CONFIG_NET_ICMPv6_SOCKET=y need to select CONFIG_SYSTEM_PING6=y and deselect CONFIG_DISABLE_POLL.
    Update NSH documention to show that ping6 is now a built in command.
    net/icmpv6:  Add icmpv6_sendto.c and icmpv6_recvfrom.c.  Initial versions are just clones from icmp/ with appropriate name changes.
    net/icmpv6:  Clone some ICMP socket logic as the beginning of support for ICMPv6 socket support.
    Rename CONFIG_NET_ICMPv6_PING to CONFIG_NET_ICMPv6_SOCKET.  Move prototype for icmpv6_ping from include/nuttx/net/icmpv6 to net/icmpv6/icmpv6.h