Perform a /usr-merge on all initrds

Since bookworm, a merged-/usr is mandatory. As we progress in finalizing the transition, we packages get simplified towards shipping all of their files below /usr and this will also affect udeb packages. If the initrds were to stay unmerged, we would expect them to break before too long. In order to avoid such breakage, /usr-merge them now.

This implementation is similar to the one proposed in debootstrap!96 (merged) and merges after the initial unpack. Merging earlier is prone to breakage when base-files starts shipping the symlinks. Note that dpkg --unpack runs preinst scripts and that this is ok, because udebs never contain preinst scripts.