[krbdev.mit.edu #7172] Credential collection doesn't include DIR subsidiary default cache

[Greg Hudson via RT]

I rewrote the patch because:

* It did unnecessary work.  We don't need a directory name in the cursor 
if we're not going to iterate over a directory, so we don't have to 
decompose the residual value and then recompose it.

* get_context_specified_file didn't conform to 
http://k5wiki.kerberos.org/wiki/Coding_style/Practices#Exception_Handlin
g or 
http://k5wiki.kerberos.org/wiki/Coding_style/Practices#Output_parameter_
handling or 
http://k5wiki.kerberos.org/wiki/Coding_style/Practices#Memory_management

* There were three test cases which all tested the same thing, but no 
test case for a nonexistent subsidiary default ccache name.