Hide `user.rootlesscontainers.*` xattrs from the container

[AkihiroSuda]

Moved from rootless-containers/runrootless#12

[penn5]

Perhaps they should all have a UUID after them, so that they can be identified on a per-container basis. This way you could even run it inside itself (though ptrace has some missing support iirc)

[AkihiroSuda]

UUID of what?

[penn5]

UUID of what?

A random UUID stored in ~/.local or somewhere