resolved: optionally, allocate DnsResourceKey objects on the stack

resolved: optionally, allocate DnsResourceKey objects on the stack

Sometimes when looking up entries in hashmaps indexed by a
DnsResourceKey it is helpful not having to allocate a full
DnsResourceKey dynamically just to use it as search key. Instead,
optionally allow allocation of a DnsResourceKey on the stack. Resource
keys allocated like that of course are subject to other lifetime cycles
than the usual Resource keys, hence initialize the reference counter to
to (unsigned) -1.

While we are at it, remove the prototype for
dns_resource_key_new_dname() which was never implemented.