util: add find_subdomain_by_sid()
    
    This function takes domain SID (doesn't have the last component)
    or object SID (have all components) and returns subdomain.
    
    The subdomain is found by comparing domain->domainid with the SID.
    
    E.g.
    domain SID: S-1-5-21-3940105347-3434501867-2690409756
    object SID: S-1-5-21-3940105347-3434501867-2690409756-513
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2034