lxc_config: mount /sys as mixed for unprivileged by default

lxc_config: mount /sys as mixed for unprivileged by default

CONTAINER_INTERFACE[0] is something systemd people call their API and
we need to adapt to it a bit, even if it means doing stupid
unnecessary things, as else systemd decides to regress and suddenly
break network stack in CT after an upgrade[1].

This mounts the parent /sys as mixed, which is:
> mount /sys as read-only but with /sys/devices/virtual/net writable.
-- man 5 lxc.container.conf

Allow users to overwrite that with a features knob, as surely some
run into other issues else and manually adding a "lxc.mount.auto"
entry in the container .conf is not an nice user experience for most.

Fixes the system regression in up to date Arch installations
introduced by[2].

[0]: https://systemd.io/CONTAINER_INTERFACE/
[1]: https://github.com/systemd/systemd/issues/15101#issuecomment-598607582
[2]: https://github.com/systemd/systemd/commit/bf331d87171b7750d1c72ab0b140a240c0cf32c3

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>