Since HEADER_DES_H has been the protector of des.h since libdes

(before SSLeay, maybe?), it's better to have that macro protect
the compatibility header des_old.h.  In the new des.h, let's use
a slightly different protecting macro.

The rationale is that there are application that might include (via
other header files, perhaps) both an old libdes des.h and OpenSSL's
des.h.  Whichever comes first would overshadow the other because of
the clash in protecting macro.  This fix solves that problem.