WL#5973: Support marking plugins as not possible

         to install or uninstall dynamically

This patch does the following:
- Step the plugin interface version.
- A flag field to the st_mysql_plugin structure that is
  used by plugins to provide basic information about the
  plugin.
- Two new flags to mark that a plugin cannot be loaded
  or unloaded dynamically (from a running server) but
  has to be installed or uninstalled offline.
- Two new error messages for reporting error when trying
  to install or uninstall a plugin marked as not instal-
  lable or not uninstallable.
- Update all plugins to initialize the new flags field
  to 0 explicitly.

This is a missing commit message for revision:

  mats.kindahl@oracle.com-20110815181211-4l94m162xs1mtrei