Handle multiple keyboard input descriptors.

Handle multiple keyboard input descriptors.
(non_keyboard_wait_mask): New variable.
(Fset_process_filter): Update non_keyboard_wait_mask
(create_process, Fopen_network_stream, deactivate_process):
(init_process, sigchld_handler): Likewise.
(wait_reading_process_input): Maybe use non_keyboard_wait_mask.
(add_keyboard_wait_descriptor):
Renamed from change_keyboard_wait_descriptor.
Clear the old descriptor's bit only the first call.
(delete_keyboard_wait_descriptor): New function.
(keyboard_bit_set): New function.
(wait_reading_process_input): Use keyboard_bit_set.
Start the loop over process descs from 0, and explicitly
ignore non-process input descs.
(init_process): Don't init keyboard_descriptor.
(keyboard_descriptor): Variable deleted.
(add_keyboard_wait_descriptor): Don't set keyboard_descriptor.
(delete_keyboard_wait_descriptor): New function.