use stdin instead of another file descriptor; make -[cnb] -r work

use stdin instead of another file descriptor; make -[cnb] -r work
on pipes

SCCS-vsn: usr.bin/tail/extern.h 5.3
SCCS-vsn: usr.bin/tail/forward.c 5.6
SCCS-vsn: usr.bin/tail/read.c 5.3
SCCS-vsn: usr.bin/tail/reverse.c 5.5
SCCS-vsn: usr.bin/tail/tail.c 5.9