The cdevpriv_dtr_t typedef was not able to be used in a function prototype
like the various d_*_t typedefs since it declared a function pointer rather
than a function. Add a new d_priv_dtor_t typedef that declares the function
and can be used as a function prototype. The previous typedef wasn't
useful outside of the cdevpriv implementation, so retire it.

The name d_priv_dtor_t was chosen to be more consistent with cdev methods
since it is commonly used in place of d_close_t even though it is not a
direct pointer in struct cdevsw.

Reviewed by: kib, imp
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D4340