Per-snap mount namespace persistence limitations

Snapd maintains a persistent mount namespace for each snap instance, that is for each snap or each parallel install of a given snap. The namespace is constructed automatically for the first invocation of application, including service, or hook that executes within the context of a particular snap. All mount namespaces are persisted as bind-mounted fragments of the nsfs, namespace filesystem, a pure-in-memory file system supported by Linux, stored over empty placeholder files stored in the /run/...