* process.c: Fix minor off-by-one issues in descriptor counts.

* process.c: Fix minor off-by-one issues in descriptor counts.

This shouldn't fix any real bugs, but it cleans up the code a bit.
(max_process_desc, max_input_desc): -1, not 0, means none.
All uses changed.
(delete_input_desc): New function.
(delete_write_fd, delete_keyboard_wait_descriptor): Use it.
(deactivate_process): Scan backwards when recomuting max_process_desc;
that should be faster.
(init_process_emacs): Initialize max_input_desc.