(Fstart_process): GCPRO current_dir before calling

(Fstart_process): GCPRO current_dir before calling
Ffind_operation_coding_system.  Encode arguments here.
(create_process): Don't encode arguments here.  Setup
src_multibyte and dst_multibyte members of struct coding.
(read_process_output): Setup src_multibyte and dst_multibyte
members of struct coding.  If the output is to multibyte buffer,
always decode the output of the process.  Adjust the
representation of 8-bit characters to the multibyteness of the
output.
(send_process): Setup coding->src_multibyte according to the
multibyteness of the source.