RESPONDER: Add sss_client_fd_handler()
    
    Currently we have 3 functions to handle client fds:
    - sec_fd_handler(): for secrets responder
    - kcm_fd_handler(): for kcm responder
    - client_fd_handler(): for all the others reponders
    
    As those functions only differ by the functions used to handle sending
    and receiving data to the fds, let's create a generic function that
    receives the specific send_fn() and recv_fn() functions.
    
    With this newly introduced function we'll be able to simply remove
    duplicated code from those 3 handlers and just call
    sss_client_fd_handler() from all of those.
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/3633
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>