Mounting tmpfs on top of a symlink fails

Using `bwrap --tmpfs PATH ...` where `PATH` happens to be a symlink to a directory fails with:

> bwrap: Can't mount tmpfs on /newroot/PATH: No such file or directory

As an example, see https://github.com/flatpak/flatpak/issues/1032

The flatpak developers worked around this by dereferencing the symlink first and mounting a tmpfs on top of the symlink's target, but perhaps bwrap should (perhaps optionally) do this itself so that other users don't need to reimplement that workaround everywhere.