Some databases might want to register their subsystems under the dn
allocated for them by monitor_register_databas. It requires that the
deferred database registration callback be called *before* the
subsystem's.
The flags are exposed using a multivalued parameter so that other
connection flags (like back-meta's) could be exported without needing
additional code.
Letting back-monitor create the subsystem entries is much more readable
than doing this manually.
Only the database is registered if needed (chain overlay has to be
updated to cope with this), the cn=Connection entry is removed in this
patch to be reinstated in the following one as part of the correct
subsystem.
The owner (overlay mostly) should already know the target subtree DN,
therefore the filter machinery can go away too.
