Move the arcfour directory from the crypto module dirs into krb. This

Move the arcfour directory from the crypto module dirs into krb.  This
directory contains the token encryption code (similar to dk, old, and
raw) which is Kerberos-specific.  The actual stream cipher lives in
enc_provider/rc4.c, which is still in the module dirs.

arcfour/arcfour-int.h contained the definitions of some structures
used only in enc_provider/rc4.c.  Move those definitions into that
source file so that everything in arcfour is at the right level of
abstraction to live in krb.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@23367 dc483132-0cff-0310-8789-dd5450dbe970