[krbdev.mit.edu #4362] kdc vague errors

[Ken Raeburn via RT]

Unless we decide to get rid of the vague-errors option altogether, I
think we should make it a run-time option in the config file, instead
of configure-time, and then add tests to make sure it works, which we
can then run even when we haven't specially configured a build tree
for the option.  We might also want to review whether we've added new
error-return paths that provide more information than we want when
vague errors are selected.

Ken