Consider TTL of CNAME record and return minimum TTL in the chain

Consider TTL of CNAME record and return minimum TTL in the chain

[BZ #13928] A DNS request consists of multiple resources combined into
a single hostent, including multiple CNAME records that may have been
assigned different TTL values. In such a case, nscd should take the
least TTL among all of the resources as the timeout for the hostent
before it is reloaded in its cache so that the hostent remains stale
in the database for the least amount of time.