libwbclient: SSSD implementation
    
    This patch implements the libwbclient API for Samba daemons and
    utilities. The main purpose is to map Active Directory users and groups
    identified by their SID to POSIX users and groups identified by their
    POSIX UIDs and GIDs respectively.
    
    The API is not fully implemented because SSSD does not support some AD
    features like WINS or NTLM. Additionally this implementation has its
    focus on the file-server use case and hence does not implement some
    features which might be needed for a domain controller use case.
    
    Some API calls are generic and independent of the backend like e.g.
    converting binary SIDs and GUIDs into a string representation and back
    or memory allocation and deallocation. These parts are taken from the
    original Samba sources together with copyright and authors. Files
    with'_sssd' as part of the name contain the SSSD related calls.
    
    Resolves: https://fedorahosted.org/sssd/ticket/1588
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>