resolved: switch from .links to postinst/rm

dpkg in some environments refuses to install the symlink and fails:

| Unpacking systemd-resolved (251.4-1) ... | dpkg: error processing archive /var/cache/apt/archives/systemd-resolved_251.4-1_amd64.deb (--unpack): | unable to make backup link of './etc/resolv.conf' before installing new version: Invalid cross-device link

So unfortunately manual handling via maintainerscripts is necessary. While there, do a backup&restore.