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?)

file_systems/fs_ops_support: Add comment for next_dirent and mark "inline."

This way, anything which includes the file will not trigger a warning
if it does not use the function.

Introduce a utility function for moving to the next dirent in read_dir.

This takes care of making sure the dirent buffer is properly aligned,
which it needs to be on some platforms (SPARC, ARM, etc.)

Change-Id: I9a6352b1e654c090a200770d51f96511ee024a99