Added facilities to handle dynamic plugins

Added facilities to handle dynamic plugins

For the purpose of demonstration, a new plugin pwd_qlty_DYN was created.
The new section in krb5.conf for dynamic plugins looks as follows
        plugin_list = PQ_DYN
        PQ_DYN = {
                plugin_api = plugin_pwd_qlty
                plugin_factory_name = plugin_dyn_factory
                plugin_factory_type = dynamic
                plugin_name = plugin_pwd_qlty_DYN
                plugin_factory_path = /var/tsitkova/Sources/pl/src/plugin_dynamic/libplugin_dynamic.so
                plugin_id = 33
        }
The test appl is server_misc.c.

git-svn-id: svn://anonsvn.mit.edu/krb5/branches/plugins@24149 dc483132-0cff-0310-8789-dd5450dbe970