Bug 611438 - Add Account Usability Control support
    
    This adds support for the account usability request and response
    controls that are used by Solaris clients.  This control allows
    one to check if an account is usable without actually performing
    a bind attempt as that entry.  The OpenDS documentation describes
    the control in greater detail.
    
    Various information about the password policy is returned in the
    response control.  To allow the new plug-in to get the password
    policy info (and to make password policy checks), I added some
    new SLAPI functions around password policies.  I did this in such
    a way that we can add support for password policy plug-ins in the
    future if we expand upon this new API.  The password policy is
    represented by an opaque Slapi_PWPolicy struct, which is currently
    just our internal pwpolicy struct.  This will allow password policy
    plug-ins to have their own struct that meets their needs if we add
    support for constructor and destructor callbacks.
    
    By default, we only allow the root DN to use this control, though
    one can set an aci in the control entry under cn=features to allow
    others to use the control.