Better ldapauth logging

[rburchell]

Description

ldapauth is a little tricky to set up. There's a lot of configuration options which make it hard to know precisely what queries are being made, and killreason is a little too simple to cover all the cases.

Describe the results you received:

It would be nice if ldapauth had a little more logging capability so that for example I can spot a failed username lookup if the bind fails, so I know which username it tried to query for, which also helps me to spot a potential misconfiguration.

Describe the results you expected:

No logging, the user just gets force-quit with a static reason.

Additional information you deem important (e.g. issue happens only occasionally):

n/a

Output of ./bin/inspircd --version:

3.9.0