Fix configuration and status codes in the MIT plugin

Fix configuration and status codes in the MIT plugin

Modify the MIT plugin to support [appdefaults] configuration in
addition to the dictionary path configured in the KDC.  Modify the
core code to return meaningful error codes instead of just 0 and 1,
adjust the Heimdal module accordingly to translate them into 0 and
1, and use that to allow the MIT code to return real error codes.

In order to support this, import more of the Kerberos portability
layer including new portability for kadmin headers.