host_callback: Fall back to AF_INET on searching with AF_UNSPEC

Previously, when an ares_gethostbyname() searched with AF_UNSPEC and the
first AF_INET6 call only returned CNAMEs, the host_callback never
retried AF_INET.

This patch makes sure than on ARES_SUCCESS, the result of AF_INET6 is
taken as authoritative only if the result contains some addresses.