(ADAPTIVE_READ_BUFFERING): New conditional.

(ADAPTIVE_READ_BUFFERING): New conditional.
(READ_OUTPUT_DELAY_INCREMENT, READ_OUTPUT_DELAY_MAX)
(READ_OUTPUT_DELAY_MAX_MAX): New constants.
(process_output_delay_count, process_output_skip): New vars.
(Vprocess_adaptive_read_buffering): New variable.
(make_process): Initialize adaptive read buffering members.
(Fstart_process): Set adaptive_read_buffering member.
(deactivate_process): Cleanup adaptive read buffering.
(wait_reading_process_input): Temporarily omit delayed
subprocesses from the set of file descriptors to read from;
adjust the select timeout if we skipped any subprocesses.
(read_process_output): Increase adaptive read buffering delay if
we read less than a full buffer; reduce delay when we read a
full buffer.
(send_process): Simplify using local Lisp_Process var.
Reset adaptive read buffering delay after write.
(init_process): Initialize process_output_delay_count and
process_output_skip.
(syms_of_process): DEFVAR_LISP Vprocess_adaptive_read_buffering.