lxc: Fix post-stop hook erroring on every container stop

The post-stop hook was set to /dev/null as a no-op. Its actual purpose (see #436) is to make LXC suppress auto-respawning the container when the guest reboots/crashes, since waydroid manages the session lifecycle itself – LXC suppresses the respawn when the post-stop hook exits non-zero, and /dev/null “achieves” this only because it is not executable.

The side effect is that the hook fails on every container stop, including a normal shutdown, logging an error that users routinely misdiagnose:

lxc-start: waydroid: utils.c: run_buffer: 569 Script exited with status 126
lxc-start: waydroid: start.c: lxc_end: 1178 Failed to run lxc.hook.post-stop for container "waydroid"

(Reproduced on LXC 7.0.0; status 126 is execve() of the non-executable /dev/null.)

Replace it with a real post-stop script that keys off LXC_TARGET: exit 0 on “stop” so a routine waydroid session stop is clean, and non-zero otherwise (reboot, or an unset value on older LXC) to preserve the respawn-suppression. The script path is substituted into the generated config in set_lxc_config(), mirroring the existing LXCARCH handling.