This commit removes the ping6 command from NSH and replaces it with the...

This commit removes the ping6 command from NSH and replaces it with the apps/system/ping6 built-in command.  The NSH ping6 command had to be removed because it violated the portable POSIX OS interface.  The apps/system/ping6 command uses the sem-standard IPPROTO_ICMP6 socket interface.

Squashed commit of the following:

    apps/system ping and ping6:  Fix a backward test.
    apps/system/ping6: ping6 logic now builds without error.
    apps/system/ping and ping6:  Minor clean-up to configuration settings.
    apps/system/ping6:  Minor clean-up to be able to configure CONFIG_SYSTEM_PING6
    apps/nshlib:  Remove support for ping6 from NSH.
    apps/system/ping6:  Add IPPROTO_ICMPv6 ping6.  Initial commit is simply a clone of the IPPROTO_ICMP ping logic.
    Rename CONFIG_NET_ICMPv6_PING to CONFIG_NET_ICMPv6_SOCKET.