procfs: Implement procfs_readdir

This patch fixes applications that read directory entries of a procfs,
e.g. ls /proc.
Modeled after ramfs.

Closes #243

Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>