Add support for ldapwhoami (RFC4532)

[a7b81a9086]

Quote from the RFC:

   The "Who am I?" operation provides a mechanism, a whoami Request, for
   the client to request that the server return the authorization
   identity it currently associates with the client.  It also provides a
   mechanism, a whoami Response, for the server to respond to that
   request.

[HarlemSquirrel]

I welcome a pull request to add this!