Fold kadm5 internal policy functions into callers

kadm5_create_policy and kadm5_modify_policy had _internal variants in
libkadm5srv (but not libkadm5clnt) which only existed to protect the
policy_refcnt field from modification over the wire.  Now that
policy_refcnt is no longer used, we don't need the separation.

Bump the library soname since this is technically an ABI change.