Enforce type checking for dns_dbversiont_t

Enforce type checking for dns_dbversiont_t

Originally, the dns_dbversion_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_dbversion_t;

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

    typedef struct dns_dbversion dns_dbversion_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_dbversion_t type.