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Use -1 to mean wait_channel is empty.
Start process loop with descriptor 0 if keyboard_descriptor is not 0.
(make_process): Initialize descriptors to -1.
(Fdelete_process): -1 means empty, in infd.
(deactivate_process): Likewise.
(process_send_signal): Likewise.
(kill_buffer_processes): Likewise.
(sigchld_handler): Likewise.
(status_notify): Likewise.
(close_process_descs): Likewise.
(create_process): Likewise.
(Fopen_network_stream): Likewise.
(send_process): Likewise.
(Fprocess_send_eof): Likewise.  Use XINT when closing outfd.
(Fset_process_filter): Use XINT to get infd value.
(wait_reading_process_input): Likewise.

(wait_reading_process_input): When bypassing select, clear Available.