Add ASN.1 codec for KKDCP's KDC-PROXY-MESSAGE

Add ASN.1 codec for KKDCP's KDC-PROXY-MESSAGE

Handle encoding and decoding [MS-KKDCP] proxy messages, including
handling of the additional length bytes.  Early versions of [MS-KKDCP]
incorrectly omit that the size of the proxied message is prepended to
the proxied message, as it is when we're using plain TCP, before
encoding the proxy-message structure.  This is fixed at least as of
version 2.1 of the spec.

[nalin@redhat.com: add tests]

ticket: 7929