(number_width): New var.

(number_width): New var.
(posixly_correct): Remove.
(struct fstatus): New struct.
(write_counts): Output fields of width number_width.
Do not worry about POSIXLY_CORRECT.
Use null file, not empty-string file, to denote stdin,
since "" is a valid file name on some hosts.
(wc, wc_file): New arg fstatus.  Use it to avoid invoking fstat
if possible.
(wc):  Avoid problems if end_pos - current_pos overflows.
Do not print odd message if stdin has a read error.
(get_input_fstatus, compute_number_width): New functions.
(main): Use them to implement the new behavior.
Ignore POSIXLY_CORRECT.