(read_process_output): Use insert_1_both.

(read_process_output): Use insert_1_both.

(read_process_output): New var `multibyte'.
Set it according to which coding system was used.

(read_process_output): Handle bytes vs chars.
(send_process): Use size_byte of OBJECT.
(Fprocess_send_string): Pass size_byte to send_process.