Allow specification of search domain in ahost

Add the "-s domain" command line option to override the search
domains.
Don't override explicitly specified search domains

Only set search domains from /etc/resolv.conf if there isn't a value
already present in the channel.
ares_parse_soa_reply: Do not leak rr_name on allocation failure

If ares_malloc_data failed, already allocated rr_name would go out of
scope.