mount-util: introduce remount_sysfs()

mount-util: introduce remount_sysfs()

This is useful when creating a new network namespace. Unlike procfs,
we need to remount sysfs, otherwise properties of the network interfaces
in the main network namespace are still accessible through the old sysfs,
e.g. /sys/class/net/eth0. All sub-mounts previously mounted on the sysfs
are moved onto the new sysfs mount.

The function will be used in later commits.