fix: fix legacy (syschdemd) mode

Quoth man mount(2)

> EINVAL
>
> A bind operation (MS_BIND) was requested where source
> referred a mount namespace magic link (i.e., a
> /proc/pid/ns/mnt magic link or a bind mount to such a
> link) and the propagation type of the parent mount of
> target was MS_SHARED, but propagation of the requested
> bind mount could lead to a circular dependency that might
> prevent the mount namespace from ever being freed.

Author: K900

modules/systemd/syschdemd/syschdemd.sh

@@ -38,6 +38,8 @@ run_in_namespace() {
 }
 
 start_systemd() {
+  mount --bind --make-private $rundir $rundir
+
   daemonize \
     -o $rundir/stdout \
     -e $rundir/stderr \