fdevname - Set errno if fdevname_r call fails; don't prepend /dev

fdevname - Set errno if fdevname_r call fails; don't prepend /dev

* Return EINVAL if the fd does not belong to a character device and
  return EBADF if the fd is not valid.

* If fdevname_r returns non-null when called from fdevname, set errno
  in fdevname to whatever *_r returned.

* Don't prepend "/dev/" string to the returned buffer, just put in
  the device name itself, without absolute path.

Reported-By: Sascha Wildner