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.