[krbdev.mit.edu #7689] kinit can create duplicate ccache in collection with default principal

[Greg Hudson via RT]

When the default cache type is collection-enabled, kinit is supposed to 
look for an existing cache matching the principal for which it will 
acquire tickets.  This happens when an explicit principal is given, but 
not when the default principal is used.  For example, in the sequence:

    KRB5CCNAME=DIR:/path/to/empty/directory
    export KRB5CCNAME
    kinit
    kinit someotherprincipal
    kdestroy
    kinit

After kdestroy is run, the collection's primary selection is 
uninitialized, and it contains a cache with tickets for $USER.  "kinit 
$USER" would search the collection and find that cache to put 
credentials in.  But because no explicit principal name was specified, 
krb5_cc_cache_match() is not used, and a second cache for $USER is 
initialized.

The code is currently structured to choose the output ccache first and 
the principal name second, because the principal name can be taken from 
the output ccache (if it exists).  We will need to alter the structure 
somehow to handle this case correctly.