file_systems/fs_ops_support: Add open_mode_to_access.

This is duplicated across multiple filesystems, and could probably be
used in more still.

Adjusted only BFS, EXT2, and NTFS in this commit, as they are the ones
which make use of fs_ops_support.h already and thus need to be modified
to avoid duplicate-definition errors.

Also tweak next_dirent to support being built under fs_shell.
(Possibly we should define ASSERT there, though?)

src/add-ons/kernel/file_systems/bfs/kernel_interface.cpp

@@ -19,8 +19,6 @@
 #include "bfs_control.h"
 #include "bfs_disk_system.h"
 
-#include <file_systems/fs_ops_support.h>
-
 // TODO: temporary solution as long as there is no public I/O requests API
 #ifndef FS_SHELL
 #	include <io_requests.h>