Added new dns_try_naptr option.

The rationale behind this is that many stacks do not implement NAPTR
correctly and rely only on DNS SRV. With misconfigured DNS servers, it
can lead to useless lookups generating potentially long delays.

This can now be disabled if not needed.
Merge pull request #69 from dsandras/ds-try-naptr2

Added new dns_try_naptr option.