fs: add __open_2 function

Nadav says:
open() has an optional third argument, "mode", which is only needed in
some cases (when the O_CREAT mode is used). As a safety feature, recent
versions of Glibc add a feature where open() with two arguments is replaced
by a call to __open_2(), which verifies it isn't called with O_CREATE.

closes #483.

Cc: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
libc: Add missing syslog.c

I forgot to include syslog.c in commit 25286ac ("libc: Stdio-based
syslog")...

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>