Stop using des_cblock * for arguments to DES functions.

This amounts to a char ** since it is a char[8] *. Evil casts mostly
resolved the fact that what was actually passed in were plain char *.
Instead, change the DES functions to use 'unsigned char *' for keys
and for input and output buffers.

Reviewed by: cem, imp
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D21389