Add support for LDAP debug output in Samba log file.

Add support for LDAP debug output in Samba log file.

Logging of the ldap libraries appears with a [LDAP] prefix
inside the samba logs. This is controlled by two new parameters:

* "ldap debug level" sets the debug level of the ldap libraries.
  It is the bit-field as understood by the openldap server.

* "ldap debug threshold" is the samba debug level at which ldap
  logging appears inside the samba logs.

This probably needs some configure tests since it makes
use of the LBER_OPT_LOG_PRINT_FN option to redirect the
debug output of the ldap libraries.

Michael
(This used to be commit 15c25990bd0bfa3145ab3c26d2bdda72993d005c)