Examples: Add support for custom nameserver to dnslookup

Updated dnslookup example to take an optional nameserver argument
for doing DNS lookup against a specific nameserver.

Task-number: QTBUG-30166
Change-Id: I9f46f9f766b56f770d2c8372e3bfad5c71023c73
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>