linux: implement ttyname as a wrapper around ttyname_r.

Big win in binary size and avoids duplicating the logic in multiple
places.

On x86_64, dropped from 1883206 to 1881790, a 1416 byte decrease.

Also changed logic to track if ttyname_buf has been allocated by
checking if it's NULL instead of tracking buflen as an additional
variable.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>