MONITOR: Fix start up with empty standard input
    
    The monitor process does not read data from standard input in.
    We can close file descriptor from stdin.
    
    [sssd] [server_stdin_handler] (0x0020): sssd: EOF on stdin - terminating
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2312
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    (cherry picked from commit 07a324c196c469d7bc47e9cb03bad919c3783ed0)