If possible don't use forwarders when priming the resolver.

If we try to fetch a record from cache and need to look into hints database we assume that the resolver is not primed and start dns_resolver_prime(). Priming query is supposed to return NSes for "." in ANSWER section and glue records for them in ADDITIONAL section, so that we can fill that info in 'regular' cache and not use hints db anymore. However, if we're using a forwarder the priming query goes through it, and if it's configured to return minimal answers we won't get the addresses of root servers in ADDITIONAL section. Since the only records for root servers we have are in hints database we'll try to prime the resolver with every single query.

This patch adds a DNS_FETCHOPT_NOFORWARD flag which avoids using forwarders if possible (that is if we have forward-first policy). Using this flag on priming fetch fixes the problem as we get the proper glue. With forward-only policy the problem is non-existent, as we'll never ask for root server addresses because we'll never have a need to query them.

Closes #752 (closed)