chg: dev: Enforce type checking for dns_dbversion_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.