MINOR: dns: functions to manage memory for a DNS resolution structure

MINOR: dns: functions to manage memory for a DNS resolution structure

A couple of new functions to allocate and free memory for a DNS
resolution structure. Main purpose is to to make the code related to DNS
more consistent.
They allocate or free memory for the structure itself. Later, if needed,
they should also allocate / free the buffers, etc, used by this structure.
They don't set/unset any parameters, this is the role of the caller.

This patch also implement calls to these function eveywhere it is
required.