mount-copybind: add rootcontext mountoption for overlayfs
If selinux is enabled, the context of the mountpoint for overlayfs
needs to be specified manually via the rootcontext option. To this
end, the required context is determined using matchpathcon(1) and
passed via the rootcontext mount option.
Additionally, if the mount source directory is created by mount-copybind
it also needs to take care that the context of the directory is correct
