core: rename BindMount structure → MountEntry

core: rename BindMount structure → MountEntry

After all, these don#t strictly encapsulate bind mounts anymore, and we are
preparing this for adding arbitrary user-defined bind mounts in a later commit,
at which point this would become really confusing. Let's clean this up, rename
the BindMount structure to MountEntry, so that it is clear that it can contain
information about any kind of mount.