mount-setup: change the system mount propagation to shared by default only at bootup

mount-setup: change the system mount propagation to shared by default only at bootup

The commit b3ac5f8cb987 has changed the system mount propagation to
shared by default, and according to the following patch:
https://github.com/opencontainers/runc/pull/208
When starting the container, the pouch daemon will call runc to execute
make-private.

However, if the systemctl daemon-reexec is executed after the container
has been started, the system mount propagation will be changed to share
again by default, and the make-private operation above will have no chance
to execute.