s3: libsmb: Add dns_lookup_list_async() - not yet used.

s3: libsmb: Add dns_lookup_list_async() - not yet used.

Take a list of hostnames and does async A and AAAA (if
supported) lookups on them. Interface compatible with
dns_lookup_list() (with the addition of one extra
parameter returning the query name list, for use inside
dsgetdcname() internals later) and we'll replace it in the next
commit. Waits for lp_get_async_dns_timeout() seconds to complete.

Commented out as not yet used.

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>