Properly handle absolute symlinks when bind-mounting.

Also move shill bind-mount to /var/run, and add /var/run/host-dbus.
Add host-dbus command to run commands in the chroot with the host system dbus.
Remove redundant mkdir in Downloads bindmount.
Launch a system dbus daemon when entering the chroot.
Kill the system dbus when unmounting the chroot.
Move bind-mounts out of /var/run to avoid conflicts with installer.
Don't fail when trying to kill the dbus daemon in unmount-chroot.
Don't hardcode resolv.conf link into prepare.sh.