Enforce type checking for dns_dbnode_t

Enforce type checking for dns_dbnode_t

Originally, the dns_dbnode_t was typedef'ed to void type.  This allowed
some flexibility, but using (void *) just removes any type-checking that
C might have.  Instead of using:

    typedef void dns_dbnode_t;

use a trick to define the type to non-existing structure:

    typedef struct dns_dbnode dns_dbnode_t;

This allows the C compilers to employ the type-checking while the
structure itself doesn't have to be ever defined because the actual
'storage' is never accessed using dns_dbnode_t type.