MINOR: dns: new snr_check_ip_callback function

MINOR: dns: new snr_check_ip_callback function

In the process of breaking links between dns_* functions and other
structures (mainly server and a bit of resolution), the function
dns_get_ip_from_response needs to be reworked: it now can call
"callback" functions based on resolution's owner type to allow modifying
the way the response is processed.

For now, main purpose of the callback function is to check that an IP
address is not already affected to an element of the same type.

For now, only server type has a callback.