Repartitioning of DNS client logic in preparation for IPv6 support

- Rename include/nuttx/net/dnsclient.h to dns.h
- Move internal DNS prototypes from dns.h to libc/netdb/lib_dns.h
- Global standard DNS definitions from libc/netdb/dns_soccket.c to dns.h
- Eliminate dns_gethostip().  This is now an internal part of gethostbyname()
- Eliminate interface dns_whois().  Not needed in this new context.