We need a sense of what the default identity is, then, with a way
to set it and list it.
The memory management model changes some, as well.
Use a bold font to indicate the current default identity in the
GUI; while here use an italic font for expired credentials.

In the process, rip out some krb4 remenants, and remove ancient
code conditional on the lack of KRB5_TC_NOTICKET.

Define USE_MESSAGE_BOX when building leash and use MessageBox().

[kaduk@mit.edu: adjust for style, flesh out commit message.]

ticket: 7253 (new)
queue: kfw
target_version: 1.10.4
tags: pullup