Allow :filter t in make-process to work as with set-process-filter

Allow :filter t in make-process to work as with set-process-filter

* src/process.c (create_process):
(create_pty):
(Fmake_pipe_process):
(Fmake_serial_process): Don't add the read_fd if the filter is t
(which means that we don't accept output from the filter).