Ticket 49051 - Enable SASL LOGIN/PLAIN support as a precursor to LDAPSSOTOKEN
    
    Bug Description:  We should allow LOGIN/PLAIN support for SASL. This is enabled
    if the user has the cyrus-sasl plain libraries installed. The reason for this
    is that LDAPSSOTOKEN will use the same userdb callbacks, so if we create them
    then we also need to support LOGIN/PLAIN else they will autodetect and will just
    be non-functional.
    
    Fix Description:  Modularise some of the bind pw verification code from bind.c
    to pw_verify.c, then make the call to it from saslbind.c
    
    https://fedorahosted.org/389/ticket/49051
    
    Author: wibrown
    
    Review by: mreynolds (Thanks!)