sys/posix: changed parameters for `fd` from `kernel_pid_t` to `int`

Using `kernel_pid_t` for the `fd` makes the opartions `posix_[open|close|read|write]` incomatible with non-RIOT implementations, e.g. using [`posix_close(fd)`](http://pubs.opengroup.org/onlinepubs/009695399/functions/close.html).
Following the POSIX standard, the `fd` must be an `int` to be portable and non specific to a system.