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.
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