Make a number of changes to the BindOperation class:

- It is now possible to create a bind operation with null arguments for the raw
  bind DN and simple credentials and have them replaced with empty ASN.1 octet
  strings.

- The ability to set the authentication type has been moved into methods for
  setting the simple credentials (which inherently sets the authentication type
  to simple) or SASL mechanism+credentials (which inherently sets the
  authentication type to SASL).

- Issue #736:  It ensures that pre-operation bind plugins get called for 
  anonymous simple binds.

- Issue #737:  It limits the set of responses that may be returned to avoid
  leaking information that may be useful to an attacker.