This patch adds the O_DIRECTORY flag to sys_open(), which causes the open
to fail with ENOTDIR if the given file is any type of file but a directory.

We need this flag as part of a correct implementation of chdir() (which
should fail on a non-directory file), and it is also required for Linux
compatibility (the O_DIRECTORY flag exists since Linux 2.1.126).

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>