isatty() is used by stdio to determine the buffering mode. Add a F_ISATTY

isatty() is used by stdio to determine the buffering mode.  Add a F_ISATTY
option to fcntl(), so that isatty() can use this rather than than the bloated
ioctl() interface.  Reducing uses of ioctl() by libc makes it easier to
constrain programs with various kinds of systrace sandboxes.
ok guenther, previously discussed as a concept with nicm