In ksu, don't stat() not-on-disk ccache residuals

In ksu, don't stat() not-on-disk ccache residuals

Don't assume that ccache residual names are filenames which we can
stat() usefully.  Instead, use helper functions to call the library
routines to try to read the default principal name from caches, and
use whether or not that succeeds as an indication of whether or not
there's a ccache in a given location.

ticket: 7728