Clarify klist -s documentation

The documentation for klist -s erroneously suggests that it doesn't
affect the exit status behavior and that it merely checks for the
existence of the ccache (only mentioning the expired ticket check at
the end).  Make it clearer and simpler, but avoid going into a lot of
detail about the nature of the expiration check.

ticket: 7806 (new)
target_version: 1.12.1
tags: pullup
Remove unused krb5_context fields

The vtbl and locate_fptrs fields were ostensibly related to the locate
pluggable interface, but weren't actually used.