libc6-<arch>: create merged-usr symlinks via preinst

There is a corner case on a merged-usr system that affects users installing and then removing a libc6- package: the top-level symlink gets removed, as the library is usually the last package to install files there.

Add a preinst snippet that, if the system is merged-usr, creates the top-level symlink used by the package. This way if it gets installed and then removed, and then installed again, everything will work. Note that this does not implement the full conversion, as that's the responsibility of the usrmerge package (moving files, etc) - this simply deals with the common case of the missing symlink.

Once all packages ship only files under /usr, hopefully in Trixie, this can be removed.

Closes: #926699

@sthibault @aurel32 gentle ping

merged

Does this code also end up in libc6.preinst (for the installation of libc6:amd64 on i386)?

No, I thought this was needed for the non-multiarch packages? Is it needed on the multiarch package too?

mentioned in merge request !12 (merged)