(Fstart_process): Use raw-text instead of emacs-mule

(Fstart_process): Use raw-text instead of emacs-mule
if enable-multibyte-characters is nil.  Initialize members
decoding_carryover and encoding_carryover of PROC.
(create_process): Adjusted for the change of encode_coding.
(Fopen_network_stream): Initialize members decoding_carryover and
encoding_carryover of PROC.
(read_process_output): Adjusted for the change of decode_coding.
(send_process): Adjusted for the change of encode_coding.