ITS#7182 Add mss_private for monitor subsystem.

ITS#7182 Add mss_private for monitor subsystem.

During initialization (and elsewhere), the subsystems have no way of
storing local data. While embedding them inside a larger strusture and
breaking out when needed (like the current back-ldap monitoring code
currently does) works, it certainly does not add to readability.

Notice that there is still no way to unregister an active subsystem.
This patch does not attempt to include such functionality while it might
actually be needed with the advent of SLAP_CONFIG_DELETE.