Copy unaligned directory info

[ChrisDenton]

Structures returned by the OS will always be correctly aligned. However, security software that impersonates the OS APIs isn't always bug free so we sometimes need to workaround issues such as wrong alignments. In this case we currently do a lot of read_unaligned and allocate a new buffer for filenames to workaround a bug in some security software that could cause issues. We can simplify by just copying unaligned data to the (no longer used) start of the buffer, which is guaranteed to be aligned because it was aligned by us.

The specific issue this was working around in a popular sandbox software has long since been fixed. However, it is possible that other such security software may suffer from the same bug so no harm in keeping the mitigation I guess. I also added a debug assert to test for this as it does indicate a bug, just not with Rust itself. It is a case that should never happen.

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[ChrisDenton]

On a more meta-note, I don't care too much about the performance of a case that should never be encountered tbh. My main motivation is to have a greater separation of alignment issues from the variable sized buffer issue because I'm hoping to create some helpers around using variable sized buffer that helps avoid accidentally doing the wrong thing (e.g. see the comment on the info reference).

[ChrisDenton]

Ah, I can't really get the lifetimes to work with the Iterator trait. Well at least not easily and I don't want to add a bunch of complexity for the sake of an edge case that should never be hit.