Convert preauth_plugin.h to new plugin framework

Convert preauth_plugin.h to new plugin framework

The preauth plugin interface was introduced in 1.6 but was never made
a public API.  In preparation for making it public in 1.10, convert it
to use the new plugin framework.  This will require changes to any
existing preauth plugins.

A number of symbols were renamed for namespace cleanliness, and
abstract types were introduced for module data and module per-request
data for better type safety.

On the consumer end (preauth2.c and kdc_preauth.c), this is a pretty
rough conversion.  Eventually we should create proper consumer APIs
with module handles, and the flat lists of preauth types should hold
pointers to module handles rather than copies of the vtables.  The
built-in preauth type handlers should then be converted to built-in
module providers linked into the consumer code (as should encrypted
challenge, since it has no external dependencies).  None of this will
impact the provider API for preauth plugins, so it can wait.

ticket: 6921

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@24970 dc483132-0cff-0310-8789-dd5450dbe970