[krbdev.mit.edu #7983] ksu without -e ignores .k5users for default principal

[Greg Hudson via RT]

My manual test for this was:

* I have tickets for ghudson at ATHENA.MIT.EDU
* Root's .k5login contains ghudson/root at ATHENA.MIT.EDU
* Root's .k5users contains ghudson at ATHENA.MIT.EDU (with or without "*" 
following)
* "ksu -n ghudson" gives me a root shell (so we know ghudson has shell 
access)
* Without the fix, "ksu" asks me for a password for ghudson/root, meaning 
it chose a less-preferred default principal because it didn't detect that 
ghudson has access.
* With the fix, "ksu" gives me a root shell on the basis of my ghudson 
tickets.