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.

ticket: 7561
target_version: 1.11.1
tags: pullup
Add kprop Python tests

Create a K5Realm.kprop_port method so test scripts can invoke kprop
usefully, and create a simple Python test script exercising the same
kprop functionality as the dejagnu suite's kprop.exp.
Desupport krb5_auth_con_setivector

ticket: 7565 (new)
Replace i_vector with cstate in auth context

Use a proper cipher state in the auth context structure, and free it
when the auth context is freed.  Simplify mk_priv/rd_priv accordingly.