Fix 0012167: Improved LDAP error handling and logging

Created a new ldap_log_error function that will call log_event with the code
and description of the last LDAP error. This is used after each failed call to
a PHP ldap function.

Improved the error handling of PHP ldap calls, some error cases were not covered

Implemented also additional improvements to LDAP logging, allowing to
fully trace what is happening throughout the LDAP authentication process.

Note: this is a manual merge of changes from commits 802bfcfc6 and
8f23dcc4, taking into consideration the specifics of 1.3.x branch.

Signed-off-by: David Hicks d@hx.id.au