debian/templates/image.postinst.in: allow postinst scripts in /usr/share/kernel/postinst.d

Allows packages to place their postinst snippets in a path in /usr instead of making them create a conf-file in /etc. If the admin wants to ignore or modify a postinst in /usr, they can create a config file of the same name in /etc and it will override or disable the postinst snippet. Disabling a script can be done by not marking it as executable or by creating a symlink to /dev/null. This tries to follow this spec: https://uapi-group.org/specifications/specs/configuration_files_specification/

Shipping content in /etc instead of /usr is problematic for packages because package upgrades become a hassle in case the user changed the file in /etc. It is not anymore easy for packages to change or delete a postinst snippet without involving writing their own postinst magic. Other parts of Debian are also moving to the pattern of: vendor ships files in /usr and admin can override them in /etc if necessary. This change allows this for the kernel postinst snippets as well.

Marking as draft as this needs more testing and discussion first. Marked as ready as this change is now being tested in the wild for MNT Reform users and we didn't spot any issues yet.