RADIUS plugin: Support multiple servers

Allow the user to specify a space-separated list of RADIUS servers.  The
"Retry" setting now specifies the number of connection retries per server.
Merge pull request #205 from weiss/master

RADIUS plugin: Add support for multiple servers
Add initial cred-prov support for change password scenario.
 - Only cred-prov code, no plugins yet.
 - Works by returning S_FALSE from GetSerialization so that Windows never
   processes the change.  Plugins will execute the change and GetSerialization
   will provide an appropriate message to the user.
Add password change pipe message.
Begin change password support in config.
 - New interface IPluginChangePassword
Configuration support for change password.
Change password support.
  - Change in parameters of IPluginChangePassword interface
Bug fix and improved messaging for change password.
Change password support in local machine plugin.
LDAP support for change password.
 - No hashing of passwords (yet)
 - No configuration of password attribute (yet)
Merge branch 'master' of https://github.com/pgina/pgina

# By David Wolff (8) and Holger Weiß (1)
# Via David Wolff
* 'master' of https://github.com/pgina/pgina:
  LDAP support for change password.  - No hashing of passwords (yet)  - No configuration of password attribute (yet)
  Change password support in local machine plugin.
  Bug fix and improved messaging for change password.
  Change password support.   - Change in parameters of IPluginChangePassword interface
  Configuration support for change password.
  Begin change password support in config.  - New interface IPluginChangePassword
  Add password change pipe message.
  Add initial cred-prov support for change password scenario.  - Only cred-prov code, no plugins yet.  - Works by returning S_FALSE from GetSerialization so that Windows never    processes the change.  Plugins will execute the change and GetSerialization    will provide an appropriate message to the user.
  RADIUS plugin: Support multiple servers