ldap_sasl_interactive_bind support

[Howard Chu <hyc@symas.com>]

I think we need to move the lutil_sasl_interact stuff into libldap. Clients 
ought to be able to use libldap's SASL support without having to #include 
Cyrus's <sasl.h> themselves. Right now they're forced to write their own 
interact handlers and the handlers must know about the SASL_CB constants etc. 
This is ugly; callers shouldn't need to know anything about what the 
underlying SASL implementation is doing.

Actually the lutil_sasl_ code should be cleaned up a bit, not used in libldap 
as-is. In particular, we should define a callback whose only purpose is to 
display a provided prompt string and retrieve user input. This callback's 
interface must have zero dependencies on <sasl.h>. We can provide a generic 
callback that writes to stderr and uses getpassphrase, but callers should be 
able to replace this with GUI dialog callbacks etc.
--
  -- Howard Chu
  CTO, Symas Corp.           http://www.symas.com
  Director, Highland Sun     http://highlandsun.com/hyc/
  Chief Architect, OpenLDAP  http://www.openldap.org/project/