Unix domain:  With these changes I can build a system with no link layer enabled, only Unix domain sockets

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