(do_command, rmt_open__, rmt_read__, rmt_lseek__, rmt_ioctl__): Use

(do_command, rmt_open__, rmt_read__, rmt_lseek__, rmt_ioctl__): Use
pid_t, size_t, ssize_t when appropriate.  Remove now-useless casts.
Use unsigned long to print *_t types, except use STRINGIFY_BIGINT for
off_t.

(get_status_string, get_status_off): New function.
(get_status): Now returns long, so that it can store ssize_t.
Invoke get_status_string to do the real work.
(rmt_read__, rmt_write__): Now returns ssize_t.  Size arg is now size_t.
(rmt_lseek__): Now returns off_t, using new get_status_off function.
(rmt_ioctl__): Convert mt_count by hand, since it might be longer than long.