Simplify and improve ksu cred verification

When verifying the user's initial credentials, don't compute a server
name and preemptively obtain creds for it.  This change allows
krb5_verify_init_creds to use any host key in the keytab, and not just
the one for the canonicalized local hostname.

[ghudson@mit.edu: rewrote commit message]

ticket: 7996 (new)
target_version: 1.13
tags: pullup