std/os/uefi: Simplify packed struct padding and default zero-initialize

Beyond adding default zero-initialization, this commit changes undefined
initialization to zero, as some cases reserved the padding and on other
cases, I've found some systems act strange when giving uninit instead of
zero even when it shouldn't be an issue, one example being
FileProtocol.Open's attributes, which *should* be ignored when not
creating a file, but ended up giving an unrelated error.
4771ac298bfifty-six committed on 1/16/2022, 7:04:39 AMparentdab4c63
7 files changedLine totals unavailable