core: make RootDirectory= and ProtectKernelModules= work

Instead of having two fields inside BindMount struct where one is stack
based and the other one is heap, use one field to store the full path
and updated it when we chase symlinks. This way we avoid dealing with
both at the same time.

This makes RootDirectory= work with ProtectHome= and ProtectKernelModules=yes

Fixes: https://github.com/systemd/systemd/issues/4567