At the moment FuChunks are sometimes mutable, and sometimes immutable, and it's
all a bit too low level for comfort.
Before we can do any kind of optimisation or verification we need plugins to
stop reading directly from the C structure. The aim here is to make FuChunk
optionally mutable without making assumptions about the memory model, and also
to be able to introspect it for the docs.
The end year is legally and functionally redundant, and more importantly causes
cherry-pick conflicts when trying to maintain old branches. Use git for history.
This is nice in theory, until you need to look at the bootloader status of the
parent, or of a different device entirely. Handle this in plugins for the few
cases we care about and stop setting or clearing IS_BOOTLOADER manually just to
get the vfuncs to be run.
Note: I do not think we want to use cleanup() for attaching devices not in
bootloader states -- as cleanup is only run at the end of the composite update.
