MIT commit d3b39a8bac6206b5ea78b0bf6a2958c1df0b0dd5 implemented
krb5_cc_remove_cred() for FILE ccaches by setting endtime to zero
and authtime to minus one and then filtering out those credentials
from get_next().
This change sets "authtime = -1" for the removed cred to permit
MIT krb5 to ignore removed credentials from a shared FILE ccache.
MIT commit 4c0838bb4c232866b95c9f2f72a55bf77cfc1308 modified the
cred_removed() check to restore compatibility with Heimdal.
Jeffrey Altman
b7e699597e