core: MultiFS infrastructure added.

MULTIFS SYNTAX:
(hd[disk number],[partition number])/path/to/file

The meaning of this_fs was changed to improve the flexibility of the support.
Now, this_fs means the file system being currently used.
root_fs was created to save the context of the main file system (where ldlinux.sys lives in).

get_fs_info is a function pointer that will be later hooked to a function from ldlinux.c32.
get_fs_info is expected to return a fs_info structure given the MultiFS path.
Wire up MultiFS support.

This patch finishes the MultiFS support.

init_multifs gets called in the main (startup) function of ldlinux.c32,
so MultiFS will be initialized automatically.

init_multifs calls enable_multifs (lives in the core) to hook get_fs_info.
Subsequent accesses will callback the get_fs_info living in ldlinux.c32.