Make kprop/kpropd work with RC4 session key

In krb5_auth_con_initivector and mk_priv/rd_priv, stop assuming that
the enctype's block size is the size of the cipher state.  Instead,
make and discard a cipher state to get the size.

(cherry picked from commit 8d01455ec9ed88bd3ccae939961a6e123bb3d45f)

ticket: 7561
version_fixed: 1.11.1
status: resolved