krb5: MIT krb5 FILE ccache remove cred compatibility

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.

lib/krb5/fcache.c

@@ -1088,6 +1088,9 @@ cred_delete(krb5_context context,
      */
     cred->times.endtime = 0;
 
+    /* For compatibility with MIT d3b39a8bac6206b5ea78b0bf6a2958c1df0b0dd5 */
+    cred->times.authtime = -1;
+
     /* ...except for config creds because we don't check their endtimes */
     if (srealm && strcmp(srealm, "X-CACHECONF:") == 0) {
 	ret = krb5_principal_set_realm(context, cred->server, "X-RMED-CONF:");