Trivial simplification, unused code removal, and .exports change

* **8db57b8** - conn_state already stores whether the connection is udp in addr.type.  This changes all checks of is_udp to the appropraite checks of addr.type.
* **892625e** - The commented code in question is already present in set_conn_state_msg_length which is called immediately below said commented code.
* **20ef6a8** - It was not a problem previously that the base64 functions were not exported from libkrb5support since they are not used outside of libkrb5support yet.  (I will need this for forthcoming pull requests, so it can be bundled there if it is felt to be more appropriate.)

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