(QCfilter_multibyte): New variable.

(QCfilter_multibyte): New variable.
(setup_process_coding_systems): New function.
(Fset_process_buffer, Fset_process_filter): Call
setup_process_coding_systems.
(Fstart_process): Initialize the member `filter_multibyte' of
struct Lisp_Process.
(create_process): Call setup_process_coding_systems.
(Fmake_network_process): New keyward `:filter-multibyte'.
Initialize the member `filter_multibyte' of struct Lisp_Process.
Call setup_process_coding_systems.
(server_accept_connection): Call setup_process_coding_systems.
(read_process_output): If the process has a filter, decide the
multibyteness of a string to given to the filter by
`filter_multibyte' member of the process.  If the process doesn't
have a filter and the result of conversion is unibyte, use
Fstring_to_multibyte (not Fstring_make_multibyte) to get the
multibyte form.
(Fset_process_coding_system): Call setup_process_coding_systems.
(Fset_process_filter_multibyte): New function.
(Fprocess_filter_multibyte_p): New function.
(syms_of_process): Intern and staticpro QCfilter_multibyte.
Defsubr Sset_process_filter_multibyte and
Sprocess_filter_multibyte_p.