Using mtools to prepare the initrd image results in an image that
exclusively uses 8.3 filenames; as per the spec, the 8.3 filenames are
stored uppercase. However, on most UNIX systems, people are used to
dealing with down-case names; therefore convert the strings to
lower-case before passing back to the user.
Implement a `shrink_to` function for a VMA. This shrinks the VMA's
region to the specified region, while recalculating file offsets and
size if the VMA is file-backed.
The `expand` function adds bytes size into the region; the check `size
>= self.size` isn't needed since this is an expanding function, not a
resizing one. To emphasise that, rename the function to `expand_by`.
Also include a bunch of unit tests to ensure that expansion by regions
with differing sizes works correctly.
Determine whether the program break needs extending with aligned logic,
but always return the requested, non-aligned, address.
Use `brk_addr` in unit tests instead.
