fix: shutdown: if lxc-stop fails, wait for socket closing with timeout

fix: shutdown: if lxc-stop fails, wait for socket closing with timeout

When trying to shutdown a hung container with `forceStop=0` (e.g. via
the Web UI), the shutdown task may run indefinitely while holding a lock
on the container config. The reason is that the shutdown subroutine
waits for the LXC command socket to close, even if the `lxc-stop`
command has failed due to timeout. This prevents other tasks (such as a
stop task) from acquiring the lock. In order to stop the container, the
shutdown task has to be explicitly killed first, which is inconvenient.
This occurs e.g. when trying to shutdown a hung CentOS 7 container (with
systemd <v232) in a cgroupv2 environment.

This fix imposes a timeout on the socket polling operation if the
`lxc-stop` command has failed. Behavior in case `lxc-stop` succeeds is
unchanged. This reintroduces some behavior from b1bad293. The timeout
duration is the given shutdown timeout, meaning that the final task
duration in the scenario above is twice the shutdown timeout.

Signed-off-by: Friedrich Weber <f.weber@proxmox.com>