Mini-OS: add concept of mount points

Mini-OS: add concept of mount points

Add the concept of mount points to Mini-OS. A mount point is a path
associated with a device pointer and an open() callback. A mount point
can be either a file (e.g. "/dev/mem") or a directory ("/var/log").

This allows to replace the special casing in the generic open()
handling with a generic mount point handling.

Prepare the open() callbacks to support creating new files by adding a
mode parameter.

Additionally add a close() prototype to include/lib.h, as it is missing
today.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>