sd-stub: make initrd passing incremental + other EFI prep work for #41543 (#41748)

This is split out of #41543 but I think makes sense of its own.

It primary does one thing: ensure that initrds installed via the Linux EFI protocol are incremental in behaviour (i.e. we read the previously set initrd and combine it with ours). So far we'd simply not install any initrds at all in this case, which would break stuff.

THis is preparatory for #41543, but is generally the better, safer behaviour.

This also contains three minor changes which are purley prep work for #41543 but shouldn't hurt in the big picture.