lib: fdtable: Define standard I/O file descriptors if CONFIG_POSIX_API

This is simplistic implementation which just redirects to (likewise
simplistic) implementation in lib/libc/newlib/libc-hooks.c. This
should be replaced with bindings to "real console", but what should
be a "real console" is so far discussed, at the RFC stage.

This implementation goes into the fdtable.c itself to keep all those
things nicely static. (This is again likely will change when we have
"real console", but again, it's so far not clear where it would
belong, so at least avoid creating random files to be deleted later).

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>