Don't include <ctype.h>; system.h already does this.

Don't include <ctype.h>; system.h already does this.
(ISODIGIT, ISSPACE): Remove; now in system.h.
(read_and): Excise ending_file_option.
(decode_header): No need to AND mode with 07777; MODE_FROM_OCT
does this now.
(from_oct): Cast ISSPACE arg to unsigned char.
(mode_from_oct): Translate modes from external to internal form.
Do not complain about unrecognized mode bits.
(decode_mode): Do not assume mode bits have traditional Unix values.