Reference random-to-key handlers through the enctype instead of the

Reference random-to-key handlers through the enctype instead of the
enc_provider, for consistency with string-to-key and the place of
implementation (other enc_provider functions are implemented in the
back end, but random-to-key handlers are in krb).  Use a single
handler for non-DES/DES3 enctypes since it's always just directly
copying the bits.  Collapse the three implementations (des, des3, and
direct) into random_to_key.c, as they're very short, and eliminate the
lib/crypto/krb/rand2key directory.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@24669 dc483132-0cff-0310-8789-dd5450dbe970