nspawn: Support idmapped mounts on homed managed home directories (#38069)

Christian made this possible in Linux 6.15 with a new system call
open_tree_attr() that combines open_tree() and mount_setattr(). Because
idmapped mounts are (rightfully) not nested, we have to do some extra
shenanigans to make source we're putting the right source uid in the
userns for any idmapped mounts that we do in nspawn.

Of course we also add the necessary boilerplate to make open_tree_attr()
available in our code and wrap open_tree_attr() and the corresponding
fallback in a new function which we then use everywhere else.