Back-monitor log functionality

[Pierangelo Masarati <masarati@aero.polimi.it>]

Hi.

I added a "cn=Log,cn=Monitor" entry to back-monitor; it shows
the current log levels that are sent to syslog in string form 
(e.g. "description: Stats" for LDAP_DEBUG_STATS and so).

The nice feature is that the values of the description attribute
can be modified via ldap_modify (and subject to ACL check).
The "modifiersName" and "modifyTimeStamp" attributes are updated
as well.

Please test this feature and the backend in general; suggestions 
for extra entries/features are welcome.

Someone is working at a "current timestamp" entry, and I'll need
to improve the statistics part (number of connections/entries/bytes/
read/sent and so on).

Pierangelo