Allow self-service for kadmin purgekeys RPC

Make the purgekeys RPC allow self-service, like the chpass and chrand
RPCs.

https://github.com/krb5/krb5/commit/9a735044d70304f22a013a81ab5c2901a64d3719
Author: Greg Hudson <ghudson@mit.edu>
Commit: 9a735044d70304f22a013a81ab5c2901a64d3719
Branch: master
 src/kadmin/server/server_stubs.c |    7 ++++---
 src/tests/t_kadmin_acl.py        |    3 +++
 2 files changed, 7 insertions(+), 3 deletions(-)