libdes (which is still used out there) or other des implementations,
the OpenSSL DES functions are renamed to begin with DES_ instead of
des_.  Compatibility routines are provided and declared by including
openssl/des_old.h.  Those declarations are the same as were in des.h
when the OpenSSL project started, which is exactly how libdes looked
at that time, and hopefully still looks today.

The compatibility functions will be removed in some future release, at
the latest in version 1.0.