Make gss_krb5_set_allowable_enctypes work for the acceptor

Make gss_krb5_set_allowable_enctypes work for the acceptor

With the addition of enctype negotiation in 1.7, a gss-krb5 acceptor
can choose an enctype for the acceptor subkey other than the one in
the keytab.  If the resulting security context will be exported and
re-imported by another gss-krb5 implementation (such as one in the
kernel), the acceptor needs a way to restrict the set of negotiated
enctypes to those supported by the other implementation.  We had that
functionality for the initiator already in the form of
gss_krb5_set_allowable_enctypes; this change makes it work for the
acceptor as well.

ticket: 6852
target_version: 1.9.1
tags: pullup

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@24603 dc483132-0cff-0310-8789-dd5450dbe970