Added extra option to enable unprivileged containers. This includes a
patch to remove the hard-coded path to `lxc-user-nic` and a new security
wrapper to set SUID to `lxc-user-nic`.
This is a feature supported out of the box by upstream and allows the
incusd service to be restarted without impacting running
instances. While this does give up a bit of reproducibility, qemu and
lxc for example, there are clear benefits in allowing the host to
apply updates without impacting instances.
Modeled after the zabbly implementation: 2a67c3e260/systemd/incus-startup.service
This will now be the default.
