Modify the preath plugin interface so that a plugin's context is

Modify the preath plugin interface so that a plugin's context is
global to all the modules within a plugin.  Also, change the
client-side interface so that the preauth plugin context (once
created) lives the lifetime of a krb5_context.  This will allow
future changes that can set plugin parameters.  The client side
request context lives the lifetime of a call to krb5_get_init_creds().

Make the sample preauth plugins buildable outside the source tree.

Fix minor memory leak in sort_krb5_padata_sequence().

Add a prototype for krb5_do_preauth_tryagain() and change the plugin
interface.

Incorporates fixes from Nalin Dahyabhai <nalin@redhat.com> for leaks
of the function table pointers (rt #4566) and fix KDC crash (rt #4567)

ticket: 4566
ticket: 4567
ticket: 4587
Target_Version: 1.6
Tags: pullup

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@18754 dc483132-0cff-0310-8789-dd5450dbe970