Networking:  IPv4 and IPv6 work together.  This fixes a bug necessary to accomplish that as well as cleaning up a couple of other issues

apps/nshlib:  Add the ping6 command to support checking IPv6 networks.  NSH logic is complete but still missing some network level support

Networking:  Correct the value returned by accept() in the case where net_lockingwait() is called.  It was returning -1 and losing the errno value.  Noted by Rony Xln

apps/netutils/telnetd:  Add protection when CONFIG_SCHED_HAVE_PARENT is enabled:  Call sigaction with SA_NOCLDWAIT so that exit status is not retained (no zombies) and block receipt of SIGCHLD so that accept is not awakened by a signal.  Iff accept() is awakened by a signal, do not do anything crazy like exit.  Most from Rony Xln

Correct name of fields in struct sockaddr_in6:  sin6_port and sin6_family, not sin_port and sin_family

apps/netutils/netlib:  All IOCTLs must use a valid address family -- even if the address family does not matter such as when getting the MAC address or the network flags

NSH library:  Disable put and get commends of TFTP client is not enabled; disable wget command if web client is not enabled

apps/nshlib:  Add logic to initialize IPv6 addresses, to display IPv6 addresses, and enough IPv6 ifconfig to allocation compilation (logic still not implemented)

Rename IPv4 netlib functions and files to make room in the namespace for corresponding IPv6 functions and files

apps/netutils/dnsclient:  Can select to be either IPv4 or IPv6, but not both (IPv6 still does not compile)

Networking:  Replace all references to the macros net_ipaddr_copy, net_ipaddr_hdrcopy, net_ipaddr_cmp, net_ipaddr_hdrcmp, and net_ipaddr_maskcmp with the appropriate IPv4 or IPv6 version of the macro (such as net_ipv4addr_copy).  The goal is to support both IPv4 and IPv6 simultaneously.  This requires that the macros be distinct and not conditionally defined to one on or the other.

Networking:  Replace all references to net_ipaddr_t with either in_addr_t on net_ipv6addr_t.  The goal is to support both IPv4 and IPv6 simultaneously.  This requires that the two types be distinct and not conditionally typedef'ed to net_ipaddr_t.

This is not the official configuration for the PX4 board and has led to confusion by NuttX users.  The board configuration also requires some ongoing maintenance and customization to support ongoing PX4 testing and evaluation.  It is best retained the PX4 repositories where it can be properly maintained and not in the upstream NuttX repository.