mount-setup: change system mount propagation to shared by default

In order to make containers work nicely out of the box it is highly
desirable to have the mount propagation mode for the root fs is set as
"shared" by default so that containers receive system mounts by default.
(See mount --make-shared for more information).

As it is unlikely that the kernel will change the default to "shared"
for this, do this early at boot-up from PID 1. Setups which prefer the
default of "private" should undo this change via invoking "mount
--make-private /" or a similar command after boot.

In the long run /etc/fstab should take the propagation mode as a mount
option like any other, so that this may be used to change the default
mode. However, if fstab is not around or doesn't list / we still should
default to shared as propagation mode, hence this change now.